Method for manufacturing semiconductor device

ABSTRACT

An embodiment of the disclosed invention is a method for manufacturing a semiconductor device, which includes the steps of forming a first insulating film; forming a source electrode, a drain electrode, and an oxide semiconductor film electrically connected to the source electrode and the drain electrode, over the first insulating film; performing heat treatment on the oxide semiconductor film to remove a hydrogen atom in the oxide semiconductor film; forming a second insulating film over the oxide semiconductor film; performing oxygen doping treatment on the second insulating film to supply an oxygen atom to the second insulating film; and forming a gate electrode in a region overlapping with the oxide semiconductor film, over the second insulating film.

TECHNICAL FIELD

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

In this specification, a semiconductor device means a device which canfunction by utilizing semiconductor characteristics, and an electroopticdevice, a semiconductor circuit, and an electronic appliance are allsemiconductor devices.

BACKGROUND ART

A technique for forming transistors using semiconductor thin filmsformed over a substrate having an insulating surface has been attractingattention. The transistors are applied to a wide range of electronicdevices such as integrated circuits (ICs) or image display devices(display devices). A silicon-based semiconductor material is widelyknown as a material for a semiconductor thin film applicable to atransistor. As another material, an oxide semiconductor has beenattracting attention.

For example, a transistor whose active layer includes an amorphous oxidecontaining indium (In), gallium (Ga), and zinc (Zn) and having anelectron carrier concentration of less than 10¹⁸/cm³ is disclosed (seePatent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528

DISCLOSURE OF INVENTION

However, when hydrogen or water, which forms an electron donor, isincluded into the oxide semiconductor in a process for manufacturing adevice, the electrical conductivity of an oxide semiconductor maychange. Such a phenomenon causes variation in the electricalcharacteristics of a transistor using the oxide semiconductor.

In view of such a problem, an object of an embodiment of the presentinvention is to provide a semiconductor device including an oxidesemiconductor, which has stable electrical characteristics and highreliability.

In a process for manufacturing a transistor including an oxidesemiconductor film, dehydration or dehydrogenation treatment isperformed by heat treatment and oxygen doping treatment is performed. Inthe process for manufacturing a transistor including an oxidesemiconductor film, at least oxygen doping treatment is performed.

An embodiment of the disclosed invention is a method for manufacturing asemiconductor device, which includes the steps of forming a firstinsulating film; forming a source electrode, a drain electrode, and anoxide semiconductor film electrically connected to the source electrodeand the drain electrode, over the first insulating film; performing heattreatment on the oxide semiconductor film to remove a hydrogen atom inthe oxide semiconductor film; forming a second insulating film over theoxide semiconductor film; performing oxygen doping treatment on thesecond insulating film to supply an oxygen atom to the second insulatingfilm; and forming a gate electrode in a region overlapping with theoxide semiconductor film, over the second insulating film.

An embodiment of the disclosed invention is a method for manufacturing asemiconductor device, which includes the steps of: forming a firstinsulating film containing an oxygen atom as its component; forming asource electrode, a drain electrode, and an oxide semiconductor filmelectrically connected to the source electrode and the drain electrode,over the first insulating film; performing heat treatment on the oxidesemiconductor film to remove a hydrogen atom in the oxide semiconductorfilm; performing oxygen doping treatment on the oxide semiconductor filmto supply an oxygen atom in the oxide semiconductor film; forming asecond insulating film containing an oxygen atom as its component overthe oxide semiconductor film; performing oxygen doping treatment on thesecond insulating film to supply an oxygen atom to the second insulatingfilm; and forming a gate electrode in a region overlapping with theoxide semiconductor film, over the second insulating film.

In the above method, the oxygen doping treatments may also be performedon the oxide semiconductor film and the second insulating film so thatthe oxide semiconductor film contains an oxygen atom at a proportiongreater than a stoichimetric proportion of the oxide semiconductor filmand less than double of the stoichimetric proportion. The oxygen dopingtreatment may be performed on the second insulating film so that thesecond insulating film contains an oxygen atom at a proportion greaterthan a stoichimetric proportion of the second insulating film and lessthan double of the stoichimetric proportion. An insulating filmcontaining a constituent element of the oxide semiconductor film may beformed as the first insulating film or the second insulating film. Asthe first insulating film or the second insulating film, a stacked filmof a third insulating film containing a constituent element of an oxidesemiconductor film and being in contact with the oxide semiconductorfilm, and a fourth insulating film containing a different element fromthe constituent element of the third insulating film may be formed. Atthis time, the third insulating film is formed so as to be sandwichedbetween the oxide semiconductor film and the fourth insulating film. Asthe first insulating film or the second insulating film, an insulatingfilm containing gallium oxide may be formed. As the first insulatingfilm or the second insulating film, a stacked film of a third insulatingfilm containing gallium oxide and being in contact with the oxidesemiconductor film, and a fourth insulating film containing a differentmaterial from gallium oxide may be formed. At this time, the thirdinsulating film is formed so as to be sandwiched between the oxidesemiconductor film and the fourth insulating film. An insulating filmcontaining nitrogen may be formed to cover the gate electrode. Note thatin this specification, the term “gallium oxide” means that oxygen andgallium are included as components and is not limited to a state asgallium oxide unless otherwise specified. For example, “an insulatingfilm containing gallium oxide” can be regarded as “an insulating filmcontaining oxygen and gallium.”

Further, in the above structure, an insulating film containing nitrogenmay be formed to cover the gate electrode. In the case where aninsulating film includes silicon nitride or the like which does notcontain hydrogen or contains an extremely small amount of hydrogen isused, oxygen which is added can be prevented from being released to theoutside, and further, hydrogen or water from the outside can beprevented from being mixed. For this reason, the insulating film isimportant.

Note that the above-described “oxygen doping” means that oxygen (whichincludes at least one of an oxygen radical, an oxygen atom, and anoxygen ion) is added to a bulk. Note that the term “bulk” is used inorder to clarify that oxygen is added not only to a surface of a thinfilm but also to the inside of the thin film In addition, “oxygendoping” includes “oxygen plasma doping” in which oxygen which is made tobe plasma is added to a bulk.

By the oxygen doping treatment, oxygen exists in at least one of theoxide semiconductor film (a bulk thereof), the insulating film (a bulkthereof), and an interface between the oxide semiconductor film and theinsulating film at an amount which is greater than a stoichiometricproportion. The amount of oxygen is preferably greater than thestoichiometric proportion and less than four times of the stoichiometricproportion, more preferably greater than the stoichiometric proportionand less than double of the stoichiometric proportion. Here, an oxideincluding excessive oxygen whose amount is greater than thestoichiometric proportion refers to, for example, an oxide whichsatisfies 2g>3a+3b+2c+4d+3e+2f, where the oxide is represented asIn_(a)Ga_(b)Zn_(c)Si_(d)Al_(e)Mg_(f)O_(g) (a, b, c, d, e, f, g≧0). Notethat oxygen which is added by the oxygen doping treatment may existbetween lattices of the oxide semiconductor.

In addition, oxygen is added so that the amount of the added oxygen islarger than at least the amount of hydrogen in the dehydrated ordehydrogenated oxide semiconductor. When the amount of the added oxygenis larger than that of hydrogen, the oxygen is diffused and reacts withhydrogen which causes instability, so that hydrogen can be fixed (madeto be an immovable ion). In other words, reduction in reliability can beprevented. In addition, with excessive oxygen, variation in a thresholdvoltage Vth caused by oxygen deficiency can be reduced and the amount ofshift ΔVth of the threshold voltage can be reduced.

Note that oxygen whose amount is equal to the above-described amountpreferably exists in two or more of the oxide semiconductor film (thebulk), the insulating film (the bulk), and the interface between theoxide semiconductor film and the insulating film.

Note that in the case where an oxide semiconductor has no oxygen defect(oxygen deficiency), the amount of oxygen included in the oxidesemiconductor may be equal to the stoichiometric proportion. However, inorder to secure reliability, for example, to suppress variation in thethreshold voltage of a transistor, an oxide semiconductor preferablyincludes oxygen whose amount is greater than the stoichiometricproportion. Similarly, in the case where an oxide semiconductor has nodefect (oxygen deficiency), the base film is not necessarily aninsulating film containing excessive oxygen. However, in order to securereliability, for example, to suppress variation in the threshold voltageof a transistor, the base film is preferably the insulating filmcontaining excessive oxygen, considering that oxygen deficiency mayoccur in the oxide semiconductor film.

Here, a state in which oxygen is added to the bulk by theabove-described “oxygen plasma doping” treatment is described. Note thatwhen oxygen doping treatment is performed on an oxide semiconductor filmcontaining oxygen as one component, it is generally difficult to checkan increase or a decrease of the oxygen concentration. Therefore, here,an effect of oxygen doping treatment was confirmed with the use of asilicon wafer.

Oxygen doping treatment was performed with the use of an inductivelycoupled plasma (ICP) method. Conditions thereof are as follows: the ICPpower was 800 W; the RF bias power was 300 W or 0 W; the pressure was1.5 Pa; the gas flow rate of oxygen gas was 75 sccm; and the substratetemperature was 70° C. FIG. 15 shows an oxygen concentration profile inthe depth direction of the silicon wafer measured by secondary ion massspectrometry (SIMS). In FIG. 15, the vertical axis represents an oxygenconcentration; the horizontal axis represents a depth from a surface ofthe silicon wafer.

It can be confirmed from FIG. 15 that oxygen is added in either of caseswhere the RF bias power is 0 W or the RF bias power is 300 W. Inaddition, in the case where the RF bias power is 300 W, oxygen is addedmore deeply as compared to the case of the RF bias power of 0 W.

Next, FIGS. 16A and 16B show results of observation of a cross sectionof the silicon wafer before and after the oxygen doping treatment byscanning transmission electron microscopy (STEM). FIG. 16A is a STEMimage of the silicon wafer which was not subjected to the oxygen dopingtreatment. FIG. 16B is a STEM image of the silicon wafer which wassubjected to the oxygen doping treatment at the RF bias voltage of 300W. As shown in FIG. 16B, it can be found that an oxygen-highly-dopedregion is formed in the silicon wafer by the oxygen doping.

As described above, it is shown that oxygen is added to the siliconwafer by performing oxygen doping on the silicon wafer. From thisresult, it is natural that oxygen can be added to an oxide semiconductorfilm by performing oxygen doping on the oxide semiconductor film.

The effect of the above structure which is an embodiment of thedisclosed invention can be easily understood as follows. Note that thebelow description is just one consideration.

When a positive voltage is applied to the gate electrode, an electricfield is generated from a gate electrode side of the oxide semiconductorfilm to a back channel side (the opposite side to the gate insulatingfilm). Therefore, hydrogen ions having positive charge which exist inthe oxide semiconductor film are transported to the back channel side,and accumulated in a region close to an interface with the insulatingfilm. The positive charge is transported from the accumulated hydrogenion to a charge trapping center (such as a hydrogen atom, water, orcontamination) in the insulating film, whereby negative charge isaccumulated in the back channel side of the oxide semiconductor film. Inother words, a parasitic channel is generated in the back channel sideof the transistor, and the threshold voltage is shifted to the negativeside, so that the transistor tends to be normally-on.

In this manner, the charge trapping center such as hydrogen or water inthe insulating film traps the positive charge and the positive charge istransported into the insulating film, whereby electrical characteristicsof the transistor vary. Accordingly, in order to suppress variation ofthe electrical characteristics of the transistor, it is important thatthere is no charge trapping center or the number of the charge trappingcenters is small in the insulating film Therefore, when an insulatingfilm is deposited, a sputtering method which causes less hydrogencontained in the deposited insulating film is preferably used. In aninsulating film deposited by a sputtering method, there is no chargetrapping center or a small number of charge trapping centers, and thetransport of positive charge does not easily occur as compared to thatin the case of using a CVD method or the like. Therefore, the shift ofthe threshold voltage of the transistor can be suppressed, and thetransistor can be normally off.

Note that in a top-gate transistor, when an oxide semiconductor film isformed over an insulating film serving as a base film and then heattreatment is performed thereon, not only water or hydrogen contained inthe oxide semiconductor film but also water or hydrogen contained in theinsulating film can be removed. Accordingly, in the insulating film,there is a small number of charge trapping centers for trapping positivecharge transported through the oxide semiconductor film. In this manner,the heat treatment for dehydration or dehydrogenation is also performedon the insulating film located below the oxide semiconductor film, inaddition to the oxide semiconductor film. Therefore, in the top-gatetransistor, the insulating film serving as a base film may be depositedby a CVD method such as a plasma CVD method.

In addition, when a negative voltage is applied to the gate electrode,an electric field is generated from the back channel side to the gateelectrode side. Thus, hydrogen ions which exist in the oxidesemiconductor film are transported to the gate insulating film side andaccumulated in a region close to the interface with the gate insulatingfilm. As a result, the threshold voltage of the transistor is shifted tothe negative side.

Note that when a voltage is kept at 0 V, the positive charge is releasedfrom the charge trapping center, so that the threshold voltage of thetransistor is shifted to the positive side, thereby returning to theinitial state, or the threshold voltage is shifted to the positive sidebeyond the initial state in some cases. These phenomena indicate theexistence of easy-to-transport ions in the oxide semiconductor film. Itcan be considered that an ion which is transported most easily is ahydrogen ion which is obtained by ionization of hydrogen that is thesmallest atom.

In addition, when the oxide semiconductor film absorbs light, a bond(also referred to as an M-H bond) of a metal element (M) and a hydrogenatom (H) in the oxide semiconductor film is broken by photoenergy. Notethat the photoenergy having a wavelength of approximately 400 nm equalsor substantially equals to the bond energy of a metal element and ahydrogen atom. When a negative gate bias is applied to a transistor inwhich a bond of a metal element and a hydrogen atom in the oxidesemiconductor film is broken, a hydrogen ion eliminated from a metalelement is attracted to a gate electrode side, so that distribution ofcharge is changed, the threshold voltage of the transistor is shifted tothe negative side, and the transistor tends to be normally on.

Note that the hydrogen ions which are transported to the interface withthe gate insulating film by light irradiation and application of anegative gate bias to the transistor are returned to the initial stateby stopping application of voltage. This can be regarded as a typicalexample of the ion transport in the oxide semiconductor film.

In order to prevent such a change of the electrical characteristics byvoltage application (BT degradation) or a change of the electricalcharacteristics by light irradiation (light degradation), it is mostimportant to remove a hydrogen atom or an impurity containing a hydrogenatom such as water thoroughly from the oxide semiconductor film tohighly purify the oxide semiconductor film. The charge density as smallas 10¹⁵ cm⁻³, or the charge per unit area which is as small as 10¹⁰ cm⁻²does not affect the transistor characteristics or very slightly affectsthem. Therefore, it is preferable that the charge density be less thanor equal to 10¹⁵ cm⁻³. Assuming that 10% of hydrogen contained in theoxide semiconductor film is transported within the oxide semiconductorfilm, it is preferable that the hydrogen concentration in the oxidesemiconductor film is less than or equal to 10¹⁶ cm⁻³. Further, in orderto prevent entrance of hydrogen from the outside after a device iscompleted, it is preferable that a silicon nitride film formed by asputtering method be used as a passivation film to cover the transistor.

Hydrogen or water can also be removed from the oxide semiconductor filmwhen an excessive amount of oxygen is added as compared to hydrogen tothe oxide semiconductor film (such that (the number of hydrogenatoms)<<(the number of oxygen radicals) or (the number of oxygen ions)).Specifically, oxygen is made to be plasma by a radio-frequency wave(RF), the bias of the substrate is increased, and an oxygen radicaland/or an oxygen ion are/is doped or added into the oxide semiconductorfilm over the substrate such that the amount of oxygen is greater thanthat of hydrogen in the oxide semiconductor film The electronegativityof oxygen is 3.0 which is larger than about 2.0, the electronegativityof a metal (Zn, Ga, In) in the oxide semiconductor film, and thus,excessive oxygen contained as compared to hydrogen abstracts hydrogenfrom the M-H group so that an OH group is formed. This OH group may forman M-O—H group with a bond to M.

It is preferable that the amount of oxygen contained in the oxidesemiconductor film be greater than the stoichiometric proportion. Forexample, in the case where an In—Ga—Zn—O-based oxide semiconductor filmis used as the oxide semiconductor film, it is far preferable that theproportion of oxygen be made to greater than the stoichiometricproportion and less than double of the stoichiometric proportion byoxygen doping or the like. For example, when the stoichiometricproportion of a single crystal of an In—Ga—Zn—O-based oxidesemiconductor is such that In:Ga:Zn:O=1:1:1:4, in an oxide semiconductorthin film whose composition is represented by InGaZnO_(x), x ispreferably greater than 4 and less than 8. Accordingly, the amount ofoxygen is greater than that of hydrogen in the oxide semiconductor film.

Photoenergy or BT stress abstracts hydrogen from the M-H group, whichcauses degradation; however, in the case where an excessive amount ofoxygen is contained as compared to the hydrogen contained in the oxidesemiconductor film, the oxygen in the oxide semiconductor film is bondedwith a hydrogen ion, so that an OH group is formed. The OH group doesnot discharge a hydrogen ion even by light irradiation or application ofBT stress on the transistor because of its high bond energy, and is noteasily transported in the oxide semiconductor film because of itsgreater mass than the mass of a hydrogen ion. Accordingly, an OH groupformed does not cause degradation of the transistor or can suppress thedegradation.

In addition, it has been confirmed that as the thickness of the oxidesemiconductor film is increased, the variation in the threshold voltageof a transistor tends to increase. It is considered that an oxygendefect in the oxide semiconductor film is one cause of the change of thethreshold voltage and increases as the thickness of the oxidesemiconductor film is increased. It is effective not only for removal ofhydrogen or water from the oxide semiconductor film but also forcompensation of an oxygen defect in the film to add an excessive amountof oxygen as compared to the hydrogen contained in an oxidesemiconductor film in a transistor according to one embodiment of thepresent invention. Accordingly, the variation in the threshold voltagecan also be suppressed in the transistor according to one embodiment ofthe present invention.

Metal oxide films each containing a component which is the same as acomponent of the oxide semiconductor film may be provided with the oxidesemiconductor film provided therebetween, which is also effective forprevention of change of the electrical characteristics. As the metaloxide film containing a component which is the same as a component ofthe oxide semiconductor film, specifically, a film containing at leastone selected from the constituent elements of the oxide semiconductorfilm is preferably used. Such a material is suitable for the oxidesemiconductor film, and therefore, provision of the metal oxide filmswith the oxide semiconductor film provided therebetween enables aninterface between the metal oxide film and the oxide semiconductor filmto be kept in an appropriate state. That is, by providing the metaloxide film using the above-described material as an insulating filmwhich is in contact with the oxide semiconductor film, accumulation ofhydrogen ions in the interface of the between the metal oxide film andthe oxide semiconductor film and in the vicinity thereof can besuppressed or prevented. Accordingly, as compared to the case whereinsulating films each containing a different component from that of theoxide semiconductor film, such as silicon oxide films, are provided withthe oxide semiconductor film provided therebetween, the hydrogenconcentration in the interface with the oxide semiconductor film, whichaffects the threshold voltage of the transistor, can be sufficientlydecreased.

A gallium oxide film is preferably used as the metal oxide film. Sincegallium oxide has a wide bandgap (Eg), by providing gallium oxide filmswith the oxide semiconductor film provided therebetween, an energybarrier is formed in the interface between the oxide semiconductor filmand the metal oxide film to prevent carrier transport in the interface.Consequently, carriers are not transported from the oxide semiconductorto the metal oxide, but are transported within the oxide semiconductorfilm. On the other hand, a hydrogen ion passes through the interfacebetween the oxide semiconductor and the metal oxide and is accumulatedin the vicinity of an interface between the metal oxide and theinsulating film. Even when the hydrogen ion is accumulated in thevicinity of the interface with the insulating film, a parasitic channelthrough which carriers can flow is not formed in the metal oxide filmsuch as a gallium oxide film, which results in no affect or a veryslight affect on the threshold voltage of the transistor. The energybarrier in the case where gallium oxide is in contact with aIn—Ga—Zn—O-based material is about 0.8 eV on the conduction band sideand is about 0.9 eV on the valence band side.

As described above, one technological idea of a transistor according toan embodiment of the present invention is to increase the amount ofoxygen contained in at least one of an insulating film in contact withan oxide semiconductor film, the oxide semiconductor film, and thevicinity of an interface between them by oxygen doping treatment.

In the case where an oxide semiconductor material which contains indiumwhose bonding strength with oxygen is relatively weak is used for theoxide semiconductor film, when the insulating film in contact with theoxide semiconductor film contains a material which has a strongerbonding strength with oxygen, such as silicon, oxygen in the oxidesemiconductor film may be abstracted by heat treatment, which may causeformation of oxygen deficiency in the vicinity of the interface of theoxide semiconductor film. However, in a transistor according to anembodiment of the disclosed invention, the formation of oxygendeficiency in the vicinity of the interface of the oxide semiconductorfilm can be suppressed by supplying excessive oxygen to the insulatingfilm in contact with the oxide semiconductor film.

Here, after the oxygen doping treatment is performed in themanufacturing process of a transistor, the amount of oxygen which isgreater than the stoichimetric proportion and is contained in the oxidesemiconductor film or the insulating film in contact with the oxidesemiconductor film may be different between layers. It can be consideredthat chemical potential of oxygen is different between the layers wherethe amount of excessive oxygen is different between them, and thedifference in the chemical potential comes to an equilibrium or asubstantial equilibrium by heat treatment or the like in themanufacturing process of the transistor. Therefore, after the oxygendoping treatment on the insulating film, heat treatment is preferablyperformed. By the heat treatment after the oxygen doping treatment,oxygen which is excessively supplied to the insulating film can bediffused and a sufficient amount of oxygen can be supplied to the oxidesemiconductor film Distribution of oxygen in the equilibrium state isdiscussed below.

The equilibrium state at a temperature T at a pressure P refers to thestate in which a Gibbs free energy of the whole of the systems, G is theminimum, which is represented by the following formula (1).

[FORMULA 1]G(N _(a) ,N _(b) ,N _(c) , . . . ,T,P)=G ⁽¹⁾(N _(a) ,N _(b) ,N _(c) , .. . ,T,P)+G ⁽²⁾(N _(a),N_(b) ,N _(c) , . . . ,T,P)+G ⁽³⁾(N _(a) ,N _(b),N _(c) , . . . ,T,P)  (1)

In the formula (1), reference symbols G⁽¹⁾, G⁽²⁾, and G⁽³⁾ denote Gibbsfree energies of layers. Reference symbols N_(a), N_(b), and N_(c)denote the number of particles, and reference symbols a, b, and c denoteparticle kinds. The Gibbs free energy changes as represented by thefollowing formula (2) when the particle a is transported from an i layerto a j layer by δN_(a) ^((j)).

$\begin{matrix}\left\lbrack {{FORMULA}\mspace{14mu} 2} \right\rbrack & \; \\{{\delta\; G} = {{{- \frac{\partial G^{(i)}}{\partial N_{a}^{(i)}}}\delta\; N_{a}^{(j)}} + {\frac{\partial G^{(j)}}{\partial N_{a\;}^{(j)}}\delta\; N_{a}^{(j)}}}} & (2)\end{matrix}$

When δG is 0 in the formula (2), or the following formula (3) issatisfied, the system is in the equilibrium state.

$\begin{matrix}\left\lbrack {{FORMULA}\mspace{14mu} 3} \right\rbrack & \; \\{\frac{\partial G^{(i)}}{\partial N_{a}^{(i)}} = \frac{\partial G^{(j)}}{\partial N_{a}^{(j)}}} & (3)\end{matrix}$

The differential of the number of particles of the Gibbs free energycorresponds to the chemical potential, and thus the chemical potentialsof particles are uniform in the layers in the equilibrium state.

In other words, when the amount of oxygen contained in the insulatingfilm in contact with the oxide semiconductor film is excessive ascompared to the oxide semiconductor film, the chemical potential ofoxygen is relatively small in the oxide semiconductor film and isrelatively large in the insulating film.

When the temperature of the whole of the systems (e.g., the oxidesemiconductor film and the insulating film in contact with the oxidesemiconductor film, here) becomes high enough to cause atom diffusion inthe layer and between the layers by heat treatment in the manufacturingprocess of the transistor, oxygen is transported so as to make thechemical potentials uniform. That is, oxygen in the insulating film istransported to the oxide semiconductor film, whereby the chemicalpotential of the insulating film is decreased and the chemical potentialof the oxide semiconductor film is increased.

In this manner, oxygen supplied excessively to the insulating film bythe oxygen doping treatment is diffused to be supplied to the oxidesemiconductor film by the following heat treatment to make the chemicalpotential of the systems to be in the equilibrium state. As describedabove, the oxygen supplied to the oxide semiconductor film is bondedwith a hydrogen ion to be an OH group, which does not cause degradationof the transistor or can suppress the degradation. In addition, supplyof oxygen to the oxide semiconductor film is effective at repairing anoxygen defect in the film.

In a transistor including a gate insulating film which is subjected todehydration or dehydrogenation treatment by heat treatment and oxygendoping treatment, the amount of change in the threshold voltage of thetransistor from before to after a bias-temperature (BT) test is small,so that the highly-reliable transistor having stable electricalcharacteristics can be obtained.

According to an embodiment of the disclosed invention, a variety ofsemiconductor devices including highly-reliable transistors havingstable electrical characteristic can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C illustrate an embodiment of a semiconductor device;

FIGS. 2A to 2G illustrate an embodiment of a method for manufacturing asemiconductor device;

FIGS. 3A to 3D each illustrate an embodiment of a semiconductor device;

FIGS. 4A to 4F illustrate an embodiment of a method for manufacturing asemiconductor device;

FIGS. 5A to 5C illustrate an embodiment of a method for manufacturing asemiconductor device;

FIGS. 6A to 6F illustrate an embodiment of a method for manufacturing asemiconductor device;

FIGS. 7A to 7C are a cross-sectional view, a top view, and a circuitdiagram of a semiconductor device, respectively;

FIGS. 8A to 8C each illustrate an embodiment of a semiconductor device;

FIG. 9 illustrates an embodiment of a semiconductor device;

FIG. 10 illustrates an embodiment of a semiconductor device;

FIG. 11 illustrates an embodiment of a semiconductor device;

FIGS. 12A and 12B illustrate an embodiment of a semiconductor device;

FIGS. 13A and 13B illustrate an electronic appliance;

FIGS. 14A to 14F illustrate electronic appliances;

FIG. 15 shows SIMS measurement results of an oxygen-doped silicon wafer;

FIGS. 16A and 16B are cross-sectional STEM images; and

FIGS. 17A and 17B are a top view and a cross-sectional view of a plasmaapparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the invention disclosed in thisspecification will be described with reference to the accompanyingdrawings. Note that the invention disclosed in this specification is notlimited to the following description, and it is easily understood bythose skilled in the art that modes and details can be variously changedwithout departing from the spirit and the scope of the invention.Therefore, the invention disclosed in this specification is notconstrued as being limited to the description of the followingembodiments.

In this specification, ordinal numbers such as “first”, “second”, and“third” are used in order to avoid confusion among components, and theterms do not limit the components numerically.

Embodiment 1

In this embodiment, a semiconductor device and a method formanufacturing the semiconductor device will be described with referenceto FIGS. 1A to 1C, FIGS. 2A to 2G, and FIGS. 3A to 3D.

<Structural Example of Semiconductor Device>

FIGS. 1A to 1C illustrate a structural example of a transistor 120.Here, FIG. 1A is a plan view, FIG. 1B is a cross-sectional view alongA-B of FIG. 1A, and FIG. 1C is a cross-sectional view along C-D of FIG.1A. Note that some of components of the transistor 120 (e.g., a gateinsulating film 110) are omitted in FIG. 1A for brevity.

The transistor 120 in FIGS. 1A to 1C includes, over a substrate 100, aninsulating film 102, a source electrode 104 a, a drain electrode 104 b,an oxide semiconductor film 108, the gate insulating film 110, and agate electrode 112.

In the transistor 120 in FIGS. 1A to 1C, the gate insulating film 110has been subjected to oxygen doping treatment and includes an oxygenexcessive region. When the gate insulating film 110 includes the oxygenexcessive region, oxygen can be prevented from being transferred fromthe oxide semiconductor film 108 to the gate insulating film 110. Inaddition, oxygen can be supplied from the gate insulating film 110 tothe oxide semiconductor film 108. Accordingly, by performing oxygendoping treatment on the gate insulating film 110, an excessive amount ofoxygen can be contained in the oxide semiconductor film 108 as comparedto hydrogen, so that the transistor 120 with improved reliability can beobtained.

<Example of Manufacturing Process of Semiconductor Device>

An example of a manufacturing process of the semiconductor device inFIGS. 1A to 1C will be described below with reference to FIGS. 2A to 2G.

First, the insulating film 102 is formed over the substrate 100 (seeFIG. 2A).

There is no particular limitation on a material of the substrate 100 aslong as the material has at least heat resistance high enough towithstand heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate can be used as the substrate 100. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like may be used as the substrate 100. Stillalternatively, any of these substrates further provided with asemiconductor element may be used as the substrate 100.

A flexible substrate may alternatively be used as the substrate 100.When a transistor is provided over the flexible substrate, thetransistor may be directly formed over the flexible substrate, or thetransistor may be formed over a different substrate and then separatedto be transferred to the flexible substrate. In order to separate thetransistor and transfer it to the flexible substrate, a separation layeris preferably formed between the different substrate and the transistor.

The insulating film 102 serves as a base. Specifically, the insulatingfilm 102 may be formed using silicon oxide, silicon nitride, aluminumoxide, aluminum nitride, gallium oxide, a mixed material thereof, or thelike. The insulating film 102 may be formed with a single-layerstructure or a layered structure using an insulating film including anyof the above materials.

There is no particular limitation on the method for forming theinsulating film 102. For example, the insulating film 102 can be formedby a deposition method such as a plasma CVD method or a sputteringmethod. A sputtering method is preferable in terms of low possibility ofentry of hydrogen, water, and the like.

Note that it is particularly preferable to form the insulating film 102with the use of an insulating material containing a component which isthe same as a component of an oxide semiconductor film formed later.Such a material is suitable for an oxide semiconductor film; thus, whenit is used for the insulating film 102, the state of the interface withthe oxide semiconductor film can be kept favorably. Here, containing “acomponent which is the same as a component of an oxide semiconductorfilm” means containing one or more of elements selected from constituentelements of the oxide semiconductor film. For example, in the case wherethe oxide semiconductor film is formed using an In—Ga—Zn—O-based oxidesemiconductor material, a gallium oxide or the like is given as such aninsulating material containing a component which is the same as acomponent of the oxide semiconductor film.

In the case where the insulating film 102 has a layered structure, it isfurther preferable to employ a layered structure of a film formed usingan insulating material containing a component which is the same as acomponent of the oxide semiconductor film (hereinafter referred to as afilm a) and a film containing a material different from a constituentmaterial of the film a (hereinafter referred to as a film b). The reasonis as follows. When the insulating film 102 has such a structure inwhich the film a and the film b are sequentially stacked from the oxidesemiconductor film side, charge is trapped preferentially in a chargetrapping center at the interface between the film a and the film b(compared with the interface between the oxide semiconductor film andthe film a) Thus, trapping of charge at the interface of the oxidesemiconductor film can be sufficiently suppressed, resulting in higherreliability of the semiconductor device.

Note that as such a layered structure, a layered structure of a galliumoxide film and a silicon oxide film, a layered structure of a galliumoxide film and a silicon nitride film, or the like can be used.

Next, a conductive film for forming the source electrode and the drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over the insulating film102 and processed to form the source electrode 104 a and the drainelectrode 104 b (see FIG. 2B). Note that the channel length L of thetransistor is determined by the distance between the edges of the sourceelectrode 104 a and the drain electrode 104 b which are formed here.

As the conductive film used for the source electrode 104 a and the drainelectrode 104 b, a metal film containing an element selected from Al,Cr, Cu, Ta, Ti, Mo, and W, a metal nitride film containing any of theabove elements as its component (e.g., a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film), or the like may beused. Alternatively, a conductive film may be used in which ahigh-melting-point metal film of Ti, Mo, W, or the like or a metalnitride film of any of these elements (a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film) may be stacked onone of or both a bottom side and a top side of a metal film of Al, Cu,or the like.

Alternatively, the conductive film used for the source electrode 104 aand the drain electrode 104 b may be formed using a conductive metaloxide. As the conductive metal oxide, indium oxide, tin oxide, zincoxide, an indium oxide-tin oxide mixed oxide (abbreviated to ITO), anindium oxide-zinc oxide mixed oxide, or any of these metal oxidematerials containing silicon oxide may be used.

The conductive film may be processed by etching with the use of a resistmask. Ultraviolet, a KrF laser light, an ArF laser light, or the like ispreferably used for light exposure for forming a resist mask for theetching.

In the case where the channel length L is less than 25 nm, the lightexposure at the time of forming the resist mask is preferably performedusing, for example, extreme ultraviolet having an extremely shortwavelength of several nanometers to several tens of nanometers. In thelight exposure using extreme ultraviolet, the resolution is high and thefocus depth is large. Thus, the channel length L of the transistorformed later can be reduced, whereby the operation speed of a circuitcan be increased.

An etching step may be performed with the use of a resist mask formedusing a so-called multi-tone mask. A resist mask formed using amulti-tone mask has a plurality of thicknesses and can be furtherchanged in shape by ashing; thus, such a resist mask can be used in aplurality of etching steps for different patterns. Therefore, a resistmask for at least two kinds of patterns can be formed using a multi-tonemask, resulting in simplification of the process

Next, an oxide semiconductor film in contact with the source electrode104 a and the drain electrode 104 b is formed over the insulating film102 and then the oxide semiconductor film is processed to form anisland-shaped oxide semiconductor film 106 (see FIG. 2C).

The oxide semiconductor film is preferably formed by a method by whichhydrogen, water, and the like do not easily enter the film, such as asputtering method. The thickness of the oxide semiconductor film ispreferably greater than or equal to 3 nm and less than or equal to 30nm. This is because the transistor might possibly be normally on whenthe oxide semiconductor film is too thick (e.g., the thickness is 50 nmor more).

As a material of the oxide semiconductor film, for example, an oxidesemiconductor material containing indium or an oxide semiconductormaterial containing indium and gallium may be used.

As a material of the oxide semiconductor film, any of the followingmaterials can be used: a four-component metal oxide such as anIn—Sn—Ga—Zn—O-based material; three-component metal oxides such as anIn—Ga—Zn—O-based material, an In—Sn—Zn—O-based material, anIn—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, and a Sn—Al—Zn—O-based material;two-component metal oxides such as an In—Zn—O-based material, aSn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-basedmaterial, a Sn—Mg—O-based material, an In—Mg—O-based material, and anIn—Ga—O-based material; and single-component metal oxides such as anIn—O-based material, a Sn—O-based material, and a Zn—O-based material.In addition, the above materials may contain silicon oxide. Here, forexample, an In—Ga—Zn—O-based material means an oxide film containingindium (In), gallium (Ga), and zinc (Zn), and there is no particularlimitation on the composition ratio thereof Further, theIn—Ga—Zn—O-based material may contain another element in addition to In,Ga, and Zn.

The oxide semiconductor film may be a thin film formed using a materialrepresented by the chemical formula, InMO₃(ZnO)_(m) (m>0). Here, Mrepresents one or more metal elements selected from Ga, Al, Mn, and Co.For example, M may be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn—O-based oxide semiconductordeposition target.

As the In—Ga—Zn—O-based oxide semiconductor deposition target, forexample, an oxide semiconductor deposition target with a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] may be used. Note that itis not necessary to limit the material and the composition ratio of thetarget to the above. For example, an oxide semiconductor depositiontarget with a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio]may alternatively be used.

In the case where an In—Zn—O-based material is used for the oxidesemiconductor, a target with the following composition ratio is used:the composition ratio of In:Zn is 50:1 to 1:2 in an atomic ratio(In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably 20:1 to 1:1 in anatomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), furtherpreferably 15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in amolar ratio). For example, a target used for the formation of anIn—Zn—O-based oxide semiconductor has the following atomic ratio:In:Zn:O is X:Y:Z, where Z>1.5X+Y.

The fill rate of the oxide semiconductor deposition target is higherthan or equal to 90% and lower than or equal to 100%, preferably, higherthan or equal to 95% and lower than or equal to 99.9%. With the use ofthe oxide semiconductor deposition target with high fill rate, a denseoxide semiconductor film can be formed.

The deposition atmosphere may be a rare gas (typically argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere containing arare gas and oxygen. Moreover, it is preferable that an atmosphere usinga high-purity gas in which impurities containing hydrogen atoms, such ashydrogen, water, a compound having a hydroxyl group, and a hydride, areremoved be used because entry of hydrogen, water, a compound having ahydroxyl group, and a hydride into the oxide semiconductor film can beprevented.

More specifically, for example, the oxide semiconductor film can beformed as follows.

First, the substrate 100 is placed in a deposition chamber kept underreduced pressure, and the substrate temperature is set to a temperaturehigher than or equal to 100° C. and lower than or equal to 600° C.,preferably higher than or equal to 200° C. and lower than or equal to400° C. This is because the concentration of an impurity contained inthe oxide semiconductor film can be reduced when deposition is performedwhile the substrate 100 is heated. This is also because damage due tosputtering can be reduced.

Then, a high-purity gas in which impurities containing hydrogen atoms,such as hydrogen and moisture, are sufficiently removed is introducedinto the deposition chamber from which remaining moisture is beingremoved, and the oxide semiconductor film is formed over the substrate100 with the use of the target. To remove moisture remaining in thedeposition chamber, an entrapment vacuum pump such as a cryopump, an ionpump, or a titanium sublimation pump is preferably used as an evacuationunit. Further, an evacuation means may be a turbo molecular pumpprovided with a cold trap. In the deposition chamber which is evacuatedwith the cryopump, a hydrogen molecule, a compound containing a hydrogenatom, such as water (H₂O), (further preferably, also a compound,containing a carbon atom), and the like are removed, whereby theconcentration of an impurity in the oxide semiconductor film formed inthe deposition chamber can be reduced.

An example of the deposition conditions is as follows: the distancebetween the substrate and the target is 100 mm, the pressure is 0.6 Pa,the direct-current (DC) power is 0.5 kW, and the deposition atmosphereis an oxygen atmosphere (the flow rate of the oxygen is 100%). Note thata pulse direct current power source is preferable because generation ofpowdery substances (also referred to as particles or dust) in depositioncan be suppressed and thickness distribution can be uniform.

The oxide semiconductor film can be processed in such a manner that amask having a desired shape is formed over the oxide semiconductor filmand then the oxide semiconductor film is etched. The mask may be formedby a method such as photolithography or an ink-jet method.

For the etching of the oxide semiconductor film, either wet etching ordry etching may be employed. Needless to say, both of them may beemployed in combination.

After that, heat treatment is performed on the oxide semiconductor film106, so that the highly-purified oxide semiconductor film 108 is formed(see FIG. 2D). Hydrogen (including water and a hydroxyl group) in theoxide semiconductor film 106 is removed through the heat treatment andthe structure of the oxide semiconductor film is rearranged, so thatdefect levels in an energy gap can be reduced. The heat treatment isperformed at a temperature higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 450° C. andlower than or equal to 600° C., or lower than the strain point of thesubstrate.

The heat treatment may be performed, for example, in such a manner thatan object to be processed is introduced into an electric furnace inwhich a resistance heating element or the like is used and heated in anitrogen atmosphere at 450° C. for an hour. During the heat treatment,the oxide semiconductor film 106 is not exposed to the air to preventthe entry of water and hydrogen.

A heat treatment apparatus is not limited to an electric furnace and maybe an apparatus for heating an object by thermal radiation or thermalconduction from a medium such as a heated gas. For example, an RTA(rapid thermal anneal) apparatus such as a GRTA (gas rapid thermalanneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus canbe used. A GRTA apparatus is an apparatus for performing heat treatmentusing a high-temperature gas. As the gas, an inert gas which does notreact with an object to be processed by heat treatment, such as nitrogenor a rare gas such as argon is used. An LRTA apparatus is an apparatusfor heating an object to be processed by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp.

For example, as the heat treatment, GRTA treatment may be performed asfollows. The object is put in an inert gas atmosphere that has beenheated, heated for several minutes, and then taken out of the inert gasatmosphere. GRTA treatment enables high-temperature heat treatment in ashort time. Moreover, GRTA treatment can be employed even when thetemperature exceeds the upper temperature limit of the object. Note thatthe inert gas may be switched to a gas containing oxygen during thetreatment. This is because the number of defect levels in an energy gapdue to oxygen deficiency can be reduced by performing the heat treatmentin an atmosphere containing oxygen.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not contain water, hydrogen, or the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into a heat treatment apparatus isgreater than or equal to 6N (99.9999%), preferably greater than or equalto 7N (99.99999%) (that is, the concentration of the impurities is lessthan or equal to 1 ppm, preferably less than or equal to 0.1 ppm).

In any case, the i-type (intrinsic) or substantially i-type oxidesemiconductor film in which impurities are reduced by the heat treatmentis formed, whereby a transistor having extremely excellentcharacteristics can be realized.

The above heat treatment can be referred to as dehydration treatment,dehydrogenation treatment, or the like because of its advantageouseffect of removing hydrogen, water, and the like. The dehydrationtreatment or dehydrogenation treatment may be performed at the timing,for example, before the oxide semiconductor film is processed to have anisland shape. Such dehydration treatment or dehydrogenation treatmentmay be conducted once or plural times.

Then, the gate insulating film 110 is formed in contact with the oxidesemiconductor film 108 so as to cover the source electrode 104 a and thedrain electrode 104 b (see FIG. 2E).

The gate insulating film 110 can be faulted in a manner similar to thatof the insulating film 102. That is, the gate insulating film 110 may beformed using silicon oxide, silicon nitride, aluminum oxide, aluminumnitride, gallium oxide, a mixed material thereof, or the like. Note thata material having a high dielectric constant, such as hafnium oxide,tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0) to which nitrogen isadded, or hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0) to which nitrogenis added may be used for the gate insulating film 110 considering thefunction of the gate insulating film of the transistor.

Note that in a similar manner to the insulating film 102, it ispreferable that an insulating material containing a component similar toa component of the oxide semiconductor film formed later be used in thegate insulating film 110. Such a material is suitable for the oxidesemiconductor film; thus, when it is used as the gate insulating film110, the state of the interface with the oxide semiconductor film can bekept favorably. Here, the “component which is the same as a component ofthe oxide semiconductor film” means containing one or more of elementsselected from constituent elements of the oxide semiconductor film. Forexample, in the case where the oxide semiconductor film is formed usingan In—Ga—Zn—O-based oxide semiconductor material, gallium oxide or thelike is given as the insulating material containing a component which isthe same as a component of the oxide semiconductor film.

As in the case of the insulating film 102, a layered structure may beemployed. In that case, it is preferable to employ a layered structureof a film formed using an insulating material containing a componentwhich is the same as a component of the oxide semiconductor film(hereinafter referred to as a film a) and a film containing a materialdifferent from that of a constituent material of the film a (hereinafterreferred to as a film b). The reason is as follows. When the gateinsulating film 110 has such a structure in which the film a and thefilm b are sequentially stacked from the oxide semiconductor film side,charge is trapped preferentially in a charge trapping center at theinterface between the film a and the film b (compared with the interfacebetween the oxide semiconductor film and the film a). Thus, trapping ofcharge at the interface of the oxide semiconductor film can besufficiently suppressed, resulting in higher reliability of thesemiconductor device.

Note that as such a layered structure, a layered structure of a galliumoxide film and a silicon oxide film, a layered structure of a galliumoxide film and a silicon nitride film, or the like may be used.

Heat treatment is preferably performed after formation of the gateinsulating film 110. The heat treatment is performed at a temperature ofhigher than or equal to 250° C. and lower than or equal to 700° C.,preferably higher than or equal to 450° C. and lower than or equal to600° C. or lower than the strain point of the substrate.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, or the like). Note that it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. Further, the purity of nitrogen,oxygen, or a rare gas introduced into a heat treatment apparatus ispreferably 6N (99.9999%) or higher (that is, the impurity concentrationis 1 ppm or lower), further preferably 7N (99.99999%) or higher (thatis, the impurity concentration is 0.1 ppm or lower).

The heat treatment in this embodiment is performed while the oxidesemiconductor film 108 and the gate insulating film 110 are in contactwith each other. Thus, oxygen, which may be reduced due to thedehydration (or dehydrogenation) treatment, can be supplied to the oxidesemiconductor film 108. In this sense, the heat treatment can also bereferred to as supply of oxygen.

Note that there is no particular limitation on the timing of the heattreatment for supply of oxygen as long as it is after formation of theoxide semiconductor film 108. For example, the heat treatment for supplyof oxygen may be performed after performing oxygen doping treatment onthe gate insulating film 110 or after forming the gate electrode. Theheat treatment for supply of oxygen may be performed following to heattreatment for dehydration or the like; heat treatment for dehydration orthe like may also serve as the heat treatment for supplying oxygen; theheat treatment for supply of oxygen may also serve as heat treatment fordehydration or the like.

As described above, the heat treatment for dehydration or the like andoxygen doping treatment or the heat treatment for supply of oxygen areapplied, whereby the oxide semiconductor film 108 can be highly purifiedso as to contain impurities as little as possible. The highly-purifiedoxide semiconductor film 108 contains extremely few (close to zero)carriers derived from a donor.

Next, the gate insulating film 110 is subjected to treatment usingoxygen 180 (also referred to as oxygen doping treatment or oxygen plasmadoping treatment) (see FIG. 2F). Here, the oxygen 180 contains at leastany of an oxygen radical, an oxygen atom, and an oxygen ion. Byperforming oxygen doping treatment on the gate insulating film 110,oxygen can be contained in the gate insulating film 110 or in thevicinity of the interface between the gate insulating film 110 and theoxide semiconductor film 108. In that case, the amount of oxygencontained in the gate insulating film 110 is greater than thestoichiometric proportion of the gate insulating film 110, or ispreferably greater than the stoichiometric proportion and less than fourtimes of the stoichiometric proportion, more preferably greater than thestoichiometric proportion and less than double of the stoichiometricproportion. Alternatively, the amount of oxygen in the gate insulatingfilm 110 can be greater than Y, or can be preferably greater than Y andless than 4Y, where the amount of oxygen in the case where the materialof the gate insulating film 110 is a single crystal is Y. Stillalternatively, the amount of oxygen contained in the gate insulatingfilm 110 can be greater than Z, and can be preferably greater than Z orless than 4Z based on the amount of oxygen Z in the insulating film inthe case where oxygen doping treatment is not performed.

For example, in the case where gallium oxide whose composition isrepresented by GaO_(x)(x>0) is used, since a single crystal of galliumoxide is Ga₂O₃, x can be greater than 1.5 and less than 6 (i.e., theamount of O is greater than 1.5 times of that of Ga and less than 6times of that of Ga). Alternatively, for example, in the case wheresilicon oxide whose composition is represented by SiO_(x) (x>0) is used,when SiO₂ (i.e., the amount of O is double of that of Si) is employed, xis greater than 2 and less than 8 (i.e., the amount of O is greater thandouble of that of Si and less than 8 times of that of Si). Note thatsuch an oxygen excessive region may exist in part of the insulating film(including its interface).

As described above, when the oxygen excessive region is formed in thegate insulating film 110 or the vicinity of the interface between thegate insulating film 110 and the oxide semiconductor film 108, oxygencan be prevented from being transferred from the oxide semiconductorfilm 108 to the gate insulating film 110. Accordingly, when oxygendoping treatment is performed on the gate insulating film 110, anexcessive amount of oxygen can be contained in the oxide semiconductorfilm 108 as compared to hydrogen.

Further, in the oxygen doping treatment, oxygen can be supplied into theoxide semiconductor film 108, the vicinity of the interface between theoxide semiconductor film 108 and the insulating film 102, or theinsulating film 102. In particular, oxygen is easily supplied into theoxide semiconductor film 108, the vicinity of the interface between theoxide semiconductor film 108 and the insulating film 102, or theinsulating film 102 in the case where the thickness of the gateinsulating film 110 or the oxide semiconductor film 108 is sufficientlysmall, in the case where an electrical bias applied to the substrate issufficiently large, or in the case where heat treatment is performedafter oxygen doping treatment.

In that case, the amount of oxygen contained in the oxide semiconductorfilm 108 is greater than the stoichiometric proportion of the oxidesemiconductor film 108, preferably greater than the stoichiometricproportion and less than double of the stoichiometric proportion.Alternatively, the amount of oxygen may be greater than Y, preferablygreater than Y and less than 2Y, where the amount of oxygen in the casewhere the material of the oxide semiconductor film 108 is a singlecrystal is Y. Still alternatively, the amount of oxygen may be greaterthan Z, preferably greater than Z and less than 2Z based on the amountof oxygen Z in the insulating film in the case where oxygen doping isnot performed. The reason of the presence of the upper limit in theabove preferable range is that the oxide semiconductor film 108 mighttake hydrogen like a hydrogen storing alloy (hydrogen storage alloy)when the amount of oxygen is too large. Note that in the oxidesemiconductor film, the amount of oxygen is larger than the amount ofhydrogen.

In the case of a material whose crystalline structure is represented byInGaO₃(ZnO)_(m) (m>0), x in InGaZnO_(x) can be greater than 4 and lessthan 8 when the crystalline structure where in is 1 (InGaZnO₄) is usedas the reference, and x in InGaZn₂O_(x) can be greater than 5 and lessthan 10 when the crystalline structure where m is 2 (InGaZn₂O₅) is usedas the reference. Such an oxygen excessive region may exist in part ofthe oxide semiconductor (including the interface).

In the oxide semiconductor film, oxygen is one of the main constituentmaterials. Thus, it is difficult to accurately estimate the oxygenconcentration of the oxide semiconductor film by a method such assecondary ion mass spectrometry (SIMS). In other words, it can be saidthat it is hard to determine whether oxygen is intentionally added tothe oxide semiconductor film.

Isotopes such as ¹⁷O or ¹⁸O exist in oxygen, and it is know that theexistence proportions of them in nature are about 0.037% and about0.204% of all the oxygen atoms. That is to say, it is possible tomeasure the concentrations of these isotopes in the oxide semiconductorfilm by a method such as SIMS; therefore, the oxygen concentration ofthe oxide semiconductor film may be able to be estimated more accuratelyby measuring the concentrations of these isotopes. Thus, theconcentrations of these isotopes may be measured to determine whetheroxygen is intentionally added to the oxide semiconductor film.

For example, when the concentration of ¹⁸O is used as the reference, D1(¹⁸O)>D2 (¹⁸O) is satisfied where D1 (¹⁸O) is the concentration of anisotope of oxygen in a region of the oxygen-doped oxide semiconductorfilm, and D2 (¹⁸O) is the concentration of an isotope of oxygen in aregion of the oxide semiconductor film which is not doped with oxygen.

It is preferable that at least part of the oxygen 180 added to the gateinsulating film 110 have dangling bonds in the oxide semiconductor filmafter being added to the semiconductor. This is because such danglingbonds are bonded with hydrogen remaining in the film so that hydrogencan be fixed (made to be immovable ions).

As described above, by performing oxygen doping treatment on the gateinsulating film 110, the oxygen excessive region is formed in the oxidesemiconductor film 108 and an excessive amount of oxygen can becontained in the oxide semiconductor film 108 as compared to hydrogen.

Further, when the oxygen excessive region is formed in the insulatingfilm 102 or in the vicinity of the interface between the insulating film102 and the oxide semiconductor film 108, oxygen can be prevented frombeing transferred from the oxide semiconductor film 108 to theinsulating film 102.

The oxygen 180 described above can be generated by a plasma generatingapparatus or an ozone generating apparatus. More specifically, forexample, an apparatus capable of etching of a semiconductor device, anapparatus capable of aching of a resist mask, or the like can be used togenerate the oxygen 180, and process the oxide semiconductor film 108.

Note that it is preferable to apply an electrical bias to the substratein order to perform oxygen doping more favorably.

Note that after the oxygen doping treatment, heat treatment ispreferably performed. By the heat treatment, an excessive amount ofoxygen can be supplied to the oxide semiconductor film as compared tohydrogen. In addition, the heat treatment may also serve as the aboveheat treatment for supply of oxygen. There is no limitation on thetiming of heat treatment for achieving the effect as long as it is afterthe oxygen doping treatment. Further, the oxygen doping treatment andthe heat treatment may be repeated. By repeatedly performing the oxygendoping treatment and the heat treatment, the transistor can have higherreliability. Note that the number of repetitions can be setappropriately.

Then, the gate electrode 112 is formed (see FIG. 2G). The gate electrode112 can be formed using a metal material such as molybdenum, titanium,tantalum, tungsten, aluminum, copper, neodymium, or scandium or an alloymaterial which contains any of these materials as its main component.Note that the gate electrode 112 may have a single-layer structure or alayered structure.

Note that an insulating film may be formed after formation of the gateelectrode 112. The insulating film may be formed using silicon oxide,silicon nitride, aluminum oxide, aluminum nitride, gallium oxide, amixed material of thereof, or the like. In particular, a silicon nitridefilm is preferable as the insulating film because added oxygen can beprevented from being released to the outside and hydrogen or the likefrom the outside can be effectively prevented from entering the oxidesemiconductor film 108. A wiring connected to the source electrode 104a, the drain electrode 104 b, the gate electrode 112, or the like may beformed.

Through the above process, the transistor 120 is formed.

<Modified Example of Semiconductor Device>

FIGS. 3A to 3D are cross-sectional views of a transistor 130, atransistor 140, a transistor 150, and a transistor 160 as modifiedexamples of the transistor 120 in FIGS. 1A to 1C.

The transistor 130 in FIG. 3A is the same as the transistor 120 in thatit includes the insulating film 102, the source electrode 104 a, thedrain electrode 104 b, the oxide semiconductor film 108, the gateinsulating film 110, and the gate electrode 112. The difference betweenthe transistor 130 and the transistor 120 is the presence of theinsulating film 114 covering the above components. That is, thetransistor 130 includes the insulating film 114. The other componentsare the same as those of the transistor 120 in FIGS. 1A to 1C; thus, thedescription of FIGS. 1A to 1C can be referred to for the detailsthereof.

The transistor 140 in FIG. 3B is the same as the transistor 120 of FIGS.1A to 1C in that it includes the above components. The differencebetween the transistor 140 and the transistor 120 is the stackingsequence of the source electrode 104 a and the drain electrode 104 b,and the oxide semiconductor film 108. That is, in the transistor 120,the source electrode 104 a and the drain electrode 104 b are formedbefore formation of the oxide semiconductor film 108, whereas in thetransistor 140, the oxide semiconductor film 108 is formed beforeformation of the source electrode 104 a and the drain electrode 104 b.The other components are the same as those in FIGS. 1A to 1C. Note thatthe transistor 140 may include the insulating film 114 like thetransistor 130.

The transistor 150 in FIG. 3C is the same as the transistor 120 of FIGS.1A to 1C in that it includes the above components. The differencebetween the transistor 150 and the transistor 120 is the insulating filmon the substrate 100 side. In other words, the transistor 150 includes astack of an insulating film 102 a and an insulating film 102 b. Theother components are the same as those in FIG. 3B.

When the layered structure of the insulating film 102 a and theinsulating film 102 b is employed in this manner, charge is trappedpreferentially in a charge trapping center at the interface between theinsulating film 102 a and the insulating film 102 b. Thus, trapping ofcharge at the interface of the oxide semiconductor film 108 can besufficiently suppressed, resulting in higher reliability of asemiconductor device.

Note that it is preferable to form the insulating film 102 b with theuse of an insulating material containing a component which is the sameas a component of the oxide semiconductor film 108 and to form theinsulating film 102 a containing a material different from a constituentmaterial of the insulating film 102 b. For example, in the case wherethe oxide semiconductor film 108 is formed using an In—Ga—Zn—O-basedoxide semiconductor material, gallium oxide or the like is given as suchan insulating material containing a component which is the same as acomponent of the oxide semiconductor film 108. In that case, a layeredstructure of a gallium oxide film and a silicon oxide film, a layeredstructure of a gallium oxide film and a silicon nitride film, or thelike may be used.

The transistor 160 in FIG. 3D is the same as the transistor 120 of FIGS.1A to 1C in that it includes the above components. The differencesbetween the transistor 160 and the transistor 120 are the insulatingfilm and the gate insulating film on the substrate 100 side. In otherwords, the transistor 160 includes a stack of the insulating film 102 aand the insulating film 102 b and a stack of a gate insulating film 110a and a gate insulating film 110 b. The other components are the same asthose in FIGS. 1A to 1C.

When the layered structure of the insulating film 102 a and theinsulating film 102 b and the layered structure of the gate insulatingfilm 110 a and the gate insulating film 110 b are employed, charge istrapped preferentially at the interface between the insulating film 102a and the insulating film 102 b and the interface between the gateinsulating film 110 a and the gate insulating film 110 b. Thus, trappingof charge at the interface of the oxide semiconductor film 108 can besufficiently suppressed, resulting in higher reliability of thesemiconductor device.

Note that it is preferable that each of the insulating film 102 b andthe gate insulating film 110 a (namely, the insulating films in contactwith the oxide semiconductor film 108) be formed with the use of aninsulating material containing a component which is the same as acomponent of the oxide semiconductor film 108, and the insulating film102 a and the gate insulating film 110 b contain materials differentfrom constituent materials of the insulating film 102 b and the gateinsulating film 110 a, respectively. For example, in the case where theoxide semiconductor film 108 is formed using an In—Ga—Zn—O-based oxidesemiconductor material, gallium oxide or the like is given as such aninsulating material containing a component which is the same as acomponent of the oxide semiconductor film 108. In this case, a layeredstructure of a gallium oxide film and a silicon oxide film, a layeredstructure of a gallium oxide film and a silicon nitride film, or thelike may be used.

The transistor according to this embodiment includes a highly-purifiedand i-type (intrinsic) oxide semiconductor film which is obtained insuch a manner that an impurity including a hydrogen atom, such ashydrogen, water, a hydroxyl group, and hydride (also referred to as ahydrogen compound), is removed from an oxide semiconductor by heattreatment, and oxygen, which might be reduced in a step for removing animpurity, is supplied so that a sufficient amount of oxygen is containedin the oxide semiconductor. The transistor including the oxidesemiconductor film which is highly purified in the above manner hassuppressed variation in the electrical characteristics such as athreshold voltage and is electrically stable.

In the case where an oxide semiconductor material which contains indiumwhose bonding strength with oxygen is relatively weak is used for theoxide semiconductor film, when the insulating film in contact with theoxide semiconductor film contains a material which has a strongerbonding strength with oxygen, such as silicon, oxygen in the oxidesemiconductor film may be abstracted by heat treatment, which may causeformation of oxygen deficiency in the vicinity of the interface of theoxide semiconductor film However, in a transistor according toembodiment of the disclosed invention, the formation of oxygendeficiency due to abstraction of oxygen from the oxide semiconductorfilm can be suppressed by supplying excessive oxygen to the insulatingfilm in contact with the oxide semiconductor film.

In particular, when the oxygen excessive region is provided in the gateinsulating film 110 by performing oxygen doping treatment on the gateinsulating film 110, oxygen can be prevented from being transferred fromthe oxide semiconductor film 108 to the gate insulating film 110. Thus,when an excessive amount of oxygen is contained in the oxidesemiconductor film as compared to hydrogen, degradation due toelectrical bias stress or heat stress can be suppressed and degradationdue to light can be reduced.

As described above, according to an embodiment of the disclosedinvention, a highly-reliable transistor can be provided.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

Embodiment 2

In this embodiment, another example of a method for manufacturing asemiconductor device will be described with reference to FIGS. 4A to 4Fand FIGS. 5A to 5C.

<Structural Example of Semiconductor Device>

The structure of a semiconductor device manufactured by themanufacturing method according to this embodiment is the same as that ofthe transistor 120 of the above embodiment. That is, the semiconductordevice includes, over the substrate 100, the insulating film 102, thesource electrode 104 a, the drain electrode 104 b, the oxidesemiconductor film 108, the gate insulating film 110, and the gateelectrode 112 (see FIGS. 1A to 1C).

As in the description in the above embodiment, in the transistor 120,the gate insulating film 110 has been subjected to oxygen dopingtreatment and includes an oxygen excessive region. When the gateinsulating film 110 includes the oxygen excessive region, oxygen can beprevented from being transferred from the oxide semiconductor film 108to the gate insulating film 110. In addition, oxygen can be suppliedfrom the gate insulating film 110 to the oxide semiconductor film 108.Further, in this embodiment, oxygen doping treatment can be directlyperformed on the oxide semiconductor film 108, and oxygen dopingtreatment can also be performed on the insulating film 102. By suchoxygen doping treatment, an excessive amount of oxygen can be containedin the oxide semiconductor film 108 as compared to hydrogen, so that thetransistor 120 with improved reliability can be obtained. Note that in asimilar to the above embodiment, transistors having different structurescan also be manufactured (see FIGS. 3A to 3D).

<Example of Manufacturing Process of Semiconductor Device>

An example of a manufacturing process of the semiconductor device willbe described below with reference to FIGS. 4A to 4F and FIGS. 5A to 5C.

First, the insulating film 102 is formed over the substrate 100 (seeFIG. 4A).

There is no particular limitation on a material of the substrate 100 aslong as the material has at least heat resistance high enough towithstand heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate can be used as the substrate 100. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like may be used as the substrate 100. Stillalternatively, any of these substrates further provided with asemiconductor element may be used as the substrate 100.

A flexible substrate may alternatively be used as the substrate 100.When a transistor is provided over the flexible substrate, thetransistor may be formed directly on the flexible substrate, or thetransistor may be formed over a different substrate and then separatedto be transferred to the flexible substrate. In order to separate thetransistor to transfer it to the flexible substrate, a separation layeris preferably formed between the different substrate and the transistor.

The insulating film 102 serves as a base. Specifically, the insulatingfilm 102 may be formed using silicon oxide, silicon nitride, aluminumoxide, aluminum nitride, gallium oxide, a mixed material of any of them,or the like. The insulating film 102 may have a single-layer structureor a layered structure using an insulating film including any of theabove materials.

There is no particular limitation on the method for forming theinsulating film 102. For example, the insulating film 102 may be formedby a deposition method such as a plasma CVD method or a sputteringmethod. A sputtering method is preferable in terms of low possibility ofentry of hydrogen, water, and the like.

Note that it is particularly preferable to form the insulating film 102with the use of an insulating material containing a component which isthe same as a component of an oxide semiconductor film formed later.Such a material is suitable for an oxide semiconductor film; thus, whenit is used for the insulating film 102, the state of the interface withthe oxide semiconductor film can be kept favorably. Here, containing “acomponent which is the same as that of an oxide semiconductor film”means containing one or more of elements selected from constituentelements of the oxide semiconductor film. For example, in the case wherethe oxide semiconductor film is formed using an In—Ga—Zn—O-based oxidesemiconductor material, gallium oxide or the like is given as such aninsulating material containing a component which is the same as acomponent of the oxide semiconductor film.

In the case where the insulating film 102 has a layered structure, it isfurther preferable to employ a layered structure of a film formed usingan insulating material containing a component which is the same as acomponent of the oxide semiconductor film (hereinafter referred to as afilm a) and a film containing a material different from a constituentmaterial of the film a (hereinafter referred to as a film b). The reasonis as follows. When the insulating film 102 has such a structure inwhich the film a and the film b are sequentially stacked from the oxidesemiconductor film side, charge is trapped preferentially in a chargetrapping center at the interface between the film a and the film b(compared with the interface between the oxide semiconductor film andthe film a). Thus, trapping of charge at the interface of the oxidesemiconductor film can be sufficiently suppressed, resulting in higherreliability of the semiconductor device.

Note that as such a layered structure, a stack of a gallium oxide filmand a silicon oxide film, a stack of a gallium oxide film and a siliconnitride film, or the like may be used.

Next, the insulating film 102 is subjected to treatment with oxygen 180a (also referred to as oxygen doping or oxygen plasma doping) (see FIG.4B). The oxygen 180 a contains at least any of an oxygen radical, anoxygen atom, and an oxygen ion. By performing oxygen doping on theinsulating film 102, oxygen can be contained in the insulating film 102and either or both in the oxide semiconductor film 108 formed lateror/and in the vicinity of the interface of the oxide semiconductor film108. In that case, the amount of oxygen contained in the insulating film102 is greater than the stoichiometric proportion of the insulating film102, or is preferably greater than the stoichiometric proportion andless than four times of the stoichiometric proportion, more preferablygreater than the stoichiometric proportion and less than double of thestoichiometric proportion. Alternatively, the amount of oxygen containedin the insulating film 102 can be greater than Y, or can be preferablygreater than Y and less than 4Y, where the amount of oxygen in the casewhere the material of the insulating film 102 is a single crystal is Y.Still alternatively, the amount of oxygen contained in the insulatingfilm 102 can be greater than Z, and can be preferably greater than Z orless than 4Z based on the amount of oxygen Z in the insulating film inthe case where oxygen doping treatment is not performed.

For example, in the case where gallium oxide whose composition isrepresented by GaO_(x)(x>0) is used, since a single crystal of galliumoxide is Ga₂O₃, x can be greater than 1.5 and less than 6 (i.e., theamount of O is greater than 1.5 times of that of Ga and less than 6times of that of Ga). Alternatively, for example, in the case wheresilicon oxide whose composition is represented by SiO_(x)(x>0) is used,when SiO₂ (i.e., the amount of O is double of that of Si) is employed, xis greater than 2 and less than 8 (i.e., the amount of O is greater thandouble of that of Si and less than 8 times of that of Si). Note thatsuch an oxygen excessive region may exist in part of the insulating film(including its interface).

As described above, when the oxygen excessive region is formed in theinsulating film 102 or the vicinity of the interface between theinsulating film 102 and the oxide semiconductor film 108 formed later,oxygen can be prevented from being transferred from the oxidesemiconductor film 108 to the insulating film 102. Accordingly, whenoxygen doping treatment is performed on the insulating film 102, anexcessive amount of oxygen can be contained in the oxide semiconductorfilm 108 as compared to hydrogen.

In addition, at least part of the oxygen 180 a added to the insulatingfilm preferably has a dangling bond in the oxide semiconductor filmafter being supplied to the oxide semiconductor. This is because, withthe dangling bond, the oxygen 180 a can be bonded with hydrogen whichremains in the film, so that the hydrogen can be fixed (made to be animmovable ion).

The oxygen 180 a can be generated by a plasma generating apparatus or anozone generating apparatus. More specifically, for example, an apparatusfor etching of a semiconductor device, an apparatus for ashing of aresist mask, or the like can be used to generate the oxygen 180 a, andthe insulating film 102 can be processed.

Note that it is preferable to apply an electrical bias to the substratein order to perform oxygen doping more favorably.

Next, a conductive film for forming the source electrode and the drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over the insulating film102 and the conductive film is processed to form the source electrode104 a and the drain electrode 104 b (see FIG. 4C). Note that the channellength L of the transistor is determined by the distance between theedges of the source electrode 104 a and the drain electrode 104 b whichare formed here.

Examples of the conductive film used for the source electrode 104 a andthe drain electrode 104 b are a metal film containing an elementselected from Al, Cr, Cu, Ta, Ti, Mo, and W, and a metal nitride filmcontaining any of the above elements as its component (e.g., a titaniumnitride film, a molybdenum nitride film, and a tungsten nitride film).Alternatively, a conductive film may be used in which ahigh-melting-point metal film of Ti, Mo, W, or the like or a metalnitride film of any of these elements (a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film) may be stacked onone of or both a bottom side and a top side of a metal film of Al, Cu,or the like.

Alternatively, the conductive film used for the source electrode 104 aand the drain electrode 104 b may be formed using a conductive metaloxide. As the conductive metal oxide, indium oxide, tin oxide, zincoxide, an indium oxide-tin oxide mixed oxide (abbreviated to ITO), anindium oxide-zinc oxide mixed oxide, or any of these metal oxidematerials containing silicon oxide may be used.

The conductive film may be processed by etching with the use of a resistmask. Ultraviolet, a KrF laser light, an ArF laser light, or the like ispreferably used for light exposure for forming a resist mask for theetching.

In the case where the channel length L is less than 25 nm, the lightexposure at the time of forming the resist mask is preferably performedusing, for example, extreme ultraviolet having an extremely shortwavelength of several nanometers to several tens of nanometers. In thelight exposure using extreme ultraviolet, the resolution is high and thefocus depth is large. Thus, the channel length L of the transistorformed later can be reduced, whereby the operation speed of a circuitcan be increased.

An etching step may be performed with the use of a resist mask formedusing a so-called multi-tone mask. A resist mask formed using amulti-tone mask has a plurality of thicknesses and can be furtherchanged in shape by ashing; thus, such a resist mask can be used in aplurality of etching steps for different patterns. Therefore, a resistmask for at least two kinds of patterns can be formed using a multi-tonemask, resulting in simplification of the process.

Next, an oxide semiconductor film in contact with the source electrode104 a and the drain electrode 104 b is formed over the insulating film102 and then the oxide semiconductor film is processed to form an oxidesemiconductor film 106 having an island shape (see FIG. 4D).

The oxide semiconductor film is preferably formed by a method by whichhydrogen, water, and the like do not easily enter the film, such as asputtering method. The thickness of the oxide semiconductor film ispreferably greater than or equal to 3 nm and less than or equal to 30nm. This is because the transistor might possibly be normally on whenthe oxide semiconductor film is too thick (e.g., the thickness is 50 nmor more).

As a material of the oxide semiconductor film, any of the followingmaterials can be used: a four-component metal oxide such as anIn—Sn—Ga—Zn—O-based material; three-component metal oxides such as anIn—Ga—Zn—O-based material, an In—Sn—Zn—O-based material, anIn—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, and a Sn—Al—Zn—O-based material;two-component metal oxides such as an In—Zn—O-based material, aSn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-basedmaterial, a Sn—Mg—O-based material, an In—Mg—O-based material, and anIn—Ga—O-based material; and single-component metal oxides such as anIn—O-based material, a Sn—O-based material, and a Zn—O-based material.In addition, the above materials may contain silicon oxide. Here, forexample, an In—Ga—Zn—O-based material means an oxide film containingindium (In), gallium (Ga), and zinc (Zn), and there is no particularlimitation on the composition ratio thereof. Further, theIn—Ga—Zn—O-based material may contain another element in addition to In,Ga, and Zn.

The oxide semiconductor film may be a thin film formed using a materialrepresented by the chemical formula, InMO₃(ZnO)_(m) (m>0). Here, Mrepresents one or more metal elements selected from Ga, Al, Mn, and Co.For example, M may be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn—O-based oxide semiconductordeposition target.

As the In—Ga—Zn—O-based oxide semiconductor deposition target, forexample, an oxide semiconductor deposition target with a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] may be used. Note that itis not necessary to limit the material and the composition ratio of thetarget to the above. For example, an oxide semiconductor depositiontarget with a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio]may alternatively be used.

In the case where an In—Zn—O-based material is used for the oxidesemiconductor, a target with the following composition ratio is used:the composition ratio of In:Zn is 50:1 to 1:2 in an atomic ratio(In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably 20:1 to 1:1 in anatomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), furtherpreferably 15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in amolar ratio). For example, a target used for the formation of anIn—Zn—O-based oxide semiconductor has the following atomic ratio:In:Zn:O is X:Y:Z, where Z>1.5X+Y.

The fill rate of the oxide semiconductor deposition target is higherthan or equal to 90% and lower than or equal to 100%, preferably, higherthan or equal to 95% and lower than or equal to 99.9%. With the use ofthe oxide semiconductor deposition target with high fill rate, a denseoxide semiconductor film can be formed.

The deposition atmosphere may be a rare gas (typically argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere containing arare gas and oxygen. Moreover, it is preferable that an atmosphere usinga high-purity gas in which impurities containing hydrogen atoms, such ashydrogen, water, a compound having a hydroxyl group, and a hydride, areremoved be used because entry of hydrogen, water, a compound having ahydroxyl group, and a hydride into the oxide semiconductor film can beprevented.

In forming the oxide semiconductor film, oxygen in the insulating film102 is supplied to the oxide semiconductor film in some cases. Whenoxygen is added to the insulating film 102 in this manner, it ispossible to form the oxide semiconductor film to which oxygen issufficiently added.

More specifically, for example, the oxide semiconductor film can beformed as follows.

First, the substrate 100 is placed in a deposition chamber kept underreduced pressure, and the substrate temperature is set to a temperaturehigher than or equal to 100° C. and lower than or equal to 600° C.,preferably higher than or equal to 200° C. and lower than or equal to400° C. This is because the concentration of an impurity contained inthe oxide semiconductor film can be reduced when deposition is performedwhile the substrate 100 is heated. This is also because damage due tosputtering can be reduced.

Then, a high-purity gas in which impurities containing hydrogen atoms,such as hydrogen and moisture, are sufficiently removed is introducedinto the deposition chamber from which remaining moisture is beingremoved, and the oxide semiconductor film is formed over the substrate100 with the use of the target. To remove moisture remaining in thedeposition chamber, an entrapment vacuum pump such as a cryopump, an ionpump, or a titanium sublimation pump is preferably used as an evacuationunit. Further, an evacuation means may be a turbo molecular pumpprovided with a cold trap. In the deposition chamber which is evacuatedwith the cryopump, a hydrogen molecule, a compound containing a hydrogenatom, such as water (H₂O), (further preferably, also a compoundcontaining a carbon atom), and the like are removed, whereby theconcentration of an impurity in the oxide semiconductor film formed inthe deposition chamber can be reduced.

An example of the deposition conditions is as follows: the distancebetween the substrate and the target is 100 mm, the pressure is 0.6 Pa,the direct-current (DC) power is 0.5 kW, and the deposition atmosphereis an oxygen atmosphere (the flow rate of the oxygen is 100%). Note thata pulse direct current power source is preferable because generation ofpowdery substances (also referred to as particles or dust) in depositioncan be prevented and thickness distribution can be uniform.

The oxide semiconductor film can be processed in such a manner that amask having a desired shape is formed over the oxide semiconductor filmand then the oxide semiconductor film is etched. The mask may be formedby a method such as photolithography or an ink-jet method.

For the etching of the oxide semiconductor film, either wet etching ordry etching may be employed. Needless to say, both of them may beemployed in combination.

After that, heat treatment is performed on the oxide semiconductor film106 so that the highly-purified oxide semiconductor film 108 is formed(see FIG. 4E). Excessive hydrogen (including water and a hydroxyl group)in the oxide semiconductor film 106 is removed through the heattreatment and the structure of the oxide semiconductor film is modified,so that defect levels in an energy gap can be reduced. Further, throughthis heat treatment, oxygen in the insulating film 102 is supplied tothe oxide semiconductor film in some cases. The heat treatment isperformed at a temperature of higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 450° C. andlower than or equal to 600° C. or lower than the strain point of thesubstrate.

The heat treatment may be performed, for example, in such a manner thatan object to be processed is introduced into an electric furnace inwhich a resistance heating element or the like is used and heated in anitrogen atmosphere at 450° C. for an hour. During the heat treatment,the oxide semiconductor film 106 is not exposed to the air to preventthe entry of water and hydrogen.

Note that a heat treatment apparatus is not limited to an electricfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a medium such as a heated gas.For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gasrapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal)apparatus can be used. A GRTA apparatus is an apparatus for performingheat treatment using a high-temperature gas. As the gas, an inert gaswhich does not react with an object to be processed by heat treatment,such as nitrogen or a rare gas such as argon is used. An LRTA apparatusis an apparatus for heating an object to be processed by radiation oflight (an electromagnetic wave) emitted from a lamp such as a halogenlamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a highpressure sodium lamp, or a high pressure mercury lamp.

For example, as the heat treatment, GRTA treatment may be performed asfollows. The object is put in an inert gas atmosphere that has beenheated, heated for several minutes, and then taken out of the inert gasatmosphere. GRTA treatment enables high-temperature heat treatment in ashort time. Moreover, GRTA treatment can be employed even when thetemperature exceeds the upper temperature limit of the object. Note thatthe inert gas may be switched to a gas including oxygen during theprocess. This is because defect levels in an energy gap due to oxygenvacancy can be reduced by performing the heat treatment in an atmospherecontaining oxygen.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not contain water, hydrogen, or the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into a heat treatment apparatus isgreater than or equal to 6 N (99.9999%), preferably greater than orequal to 7 N (99.99999%) (that is, the concentration of the impuritiesis less than or equal to 1 ppm, preferably less than or equal to 0.1ppm).

In any case, the i-type (intrinsic) or substantially i-type oxidesemiconductor film in which impurities are reduced by the heat treatmentis formed, whereby a transistor having extremely excellentcharacteristics can be realized.

The above heat treatment can be referred to as dehydration treatment,dehydrogenation treatment, or the like because of its advantageouseffect of removing hydrogen, water, and the like. The dehydrationtreatment or dehydrogenation treatment may be performed at the timing,for example, before the oxide semiconductor film is processed to have anisland shape. Such dehydration treatment or dehydrogenation treatmentmay be conducted once or plural times.

Next, the oxide semiconductor film 108 is subjected to treatment usingoxygen 180 b (see FIG. 4F). The oxygen 180 b contains at least any of anoxygen radical, an oxygen atom, and an oxygen ion. By doping the oxidesemiconductor film 108 with oxygen, the oxygen can be contained eitheror both in the oxide semiconductor film 108 or/and in the vicinity ofthe interface of the oxide semiconductor film 108.

As described above, by performing oxygen doping treatment on the oxidesemiconductor film 108, the oxygen excessive region is formed in theoxide semiconductor film 108 and an excessive amount of oxygen can becontained in the oxide semiconductor film 108 as compared to hydrogen.

It is preferable that at least part of the oxygen 180 b added to theoxide semiconductor film have dangling bonds in the oxide semiconductorfilm. This is because such dangling bonds are linked with hydrogenremaining in the film so that hydrogen can be fixed (made to beimmovable ions).

The oxygen 180 b can be generated by a plasma generating apparatus or anozone generating apparatus. More specifically, for example, an apparatusfor etching of a semiconductor device, an apparatus for ashing of aresist mask, or the like can be used to generate the oxygen 180 b, andprocess the oxide semiconductor film 108.

Note that it is preferable to apply an electrical bias to the substratein order to add oxygen more favorably.

Heat treatment (at a temperature of 150° C. to 470° C.) may be performedon the oxide semiconductor film 108 which has been subjected to oxygendoping treatment. Through the heat treatment, water, a hydroxyl group(OH), and the like generated by reaction between hydrogen and thematerial of the oxide semiconductor can be removed from the oxidesemiconductor film. The heat treatment may be performed in an atmosphereof nitrogen, oxygen, an ultra-dry air (an air where the moisture contentis 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1ppm or less, further preferably 10 ppb or less when measurement isperformed using a dew-point instrument of a cavity ring down laserspectroscopy (CRDS) system), a rare gas (e.g., argon or helium), or thelike in which moisture, hydrogen, and the like are sufficiently reduced.Further, the oxygen doping treatment and the heat treatment may berepeated. By repeatedly performing the oxygen doping treatment and theheat treatment, the transistor can have higher reliability. The numberof repetitions can be set appropriately.

Then, the gate insulating film 110 is formed in contact with part of theoxide semiconductor film 108 so as to cover the source electrode 104 aand the drain electrode 104 b (see FIG. 5A).

The gate insulating film 110 can be formed in a manner similar to thatof the insulating film 102. That is, the gate insulating film 110 may beformed using silicon oxide, silicon nitride, aluminum oxide, aluminumnitride, gallium oxide, a mixed material thereof, or the like. Note thata material having a high dielectric constant, such as hafnium oxide,tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0), hafnium silicate (HfSiO_(y) (x>0, y>0) to which nitrogen is added,or hafnium aluminate (HfSi_(x)O_(y) (x>0, y>0) to which nitrogen isadded may be used for the gate insulating film 110 considering thefunction of the gate insulating film of the transistor.

Note that in a similar manner to the insulating film 102, it ispreferable that an insulating material containing a component which isthe same as a component of the oxide semiconductor film formed later beused for the gate insulating film 110. Such a material is suitable forthe oxide semiconductor film; thus, when it is used for the gateinsulating film 110, the state of the interface between the oxidesemiconductor film and the gate insulating film 110 can be keptfavorably. Here, the “component which is the same as a component of theoxide semiconductor film” means containing one or more of metal elementsselected from components of the oxide semiconductor film. For example,in the case where the oxide semiconductor film is formed from anIn—Ga—Zn—O-based oxide semiconductor material, gallium oxide or the likeis given as the insulating material formed from a component which is thesame as a component of the oxide semiconductor film.

As in the case of the insulating film 102, a layered structure may beemployed. In that case, it is preferable to employ a layered structureof a film formed using an insulating material containing a componentwhich is the same as a component of the oxide semiconductor film(hereinafter referred to as a film a) and a film containing a materialdifferent from a constituent material of the film a (hereinafterreferred to as a film b). The reason is as follows. When the gateinsulating film 110 has such a structure in which the film a and thefilm b are sequentially stacked from the oxide semiconductor film side,charge is trapped preferentially in a charge trapping center at theinterface between the film a and the film b (compared with the interfacebetween the oxide semiconductor film and the film a). Thus, trapping ofcharge at the interface with the oxide semiconductor film can besufficiently suppressed, resulting in higher reliability of thesemiconductor device.

Note that as such a layered structure, a layered structure of a galliumoxide film and a silicon oxide film, a layered structure of a galliumoxide film and a silicon nitride film, or the like may be used.

Heat treatment is preferably performed after formation of the gateinsulating film 110. The heat treatment is performed at a temperature ofhigher than or equal to 250° C. and lower than or equal to 700° C.,preferably higher than or equal to 450° C. and lower than or equal to600° C. or lower than the strain point of the substrate.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, or the like). Note that it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. Further, the purity of nitrogen,oxygen, or a rare gas introduced into a heat treatment apparatus ispreferably 6N (99.9999%) or higher (that is, the impurity concentrationis 1 ppm or lower), further preferably 7N (99.99999%) or higher (thatis, the impurity concentration is 0.1 ppm or lower).

The heat treatment in this embodiment is performed while the oxidesemiconductor film 108 is in contact with the insulating film 102 andthe gate insulating film 110. Thus, oxygen, which may be reduced due tothe dehydration (or dehydrogenation) treatment, can be supplied from theinsulating film 102, the gate insulating film 110, or the like to theoxide semiconductor film 108. In this sense, the heat treatment can alsobe referred to as supply of oxygen.

Note that there is no particular limitation on the timing of the heattreatment for supply of oxygen as long as it is after formation of theoxide semiconductor film 108. For example, the heat treatment for supplyof oxygen may be performed after performing oxygen doping treatment onthe gate insulating film 110 or after forming the gate electrode. Theheat treatment for supply of oxygen may be performed following to heattreatment for dehydration or the like; heat treatment for dehydration orthe like may also serve as the heat treatment for supplyoffing oxygen;the heat treatment for supply of oxygen may also serve as heat treatmentfor dehydration or the like.

As described above, the heat treatment for dehydration or the like andoxygen doping treatment or the heat treatment for supply of oxygen areapplied, whereby the oxide semiconductor film 108 can be highly purifiedso as to contain impurities as little as possible. The highly-purifiedoxide semiconductor film 108 contains extremely few (close to zero)carriers derived from a donor.

Next, the gate insulating film 110 is subjected to treatment usingoxygen 180 c (also referred to as oxygen doping treatment or oxygenplasma doping treatment) (see FIG. 5B). Here, the oxygen 180 c containsat least any of an oxygen radical, an oxygen atom, and an oxygen ion. Byperforming oxygen doping treatment on the gate insulating film 110,oxygen can be contained in the gate insulating film 110 or in thevicinity of the interface between the gate insulating film 110 and theoxide semiconductor film 108. In that case, the amount of oxygencontained in the gate insulating film 110 is greater than thestoichiometric proportion of the gate insulating film 110, or ispreferably greater than the stoichiometric proportion and less than fourtimes of the stoichiometric proportion, more preferably greater than thestoichiometric proportion and less than double of the stoichiometricproportion. Alternatively, the amount of oxygen contained in the gateinsulating film 110 can be greater than Y, or can be preferably greaterthan Y and less than 4Y, where the amount of oxygen in the case wherethe material of the gate insulating film 110 is a single crystal is Y.Still alternatively, the amount of oxygen contained in the gateinsulating film 110 can be greater than Z, and can be preferably greaterthan Z or less than 4Z based on the amount of oxygen Z in the insulatingfilm in the case where oxygen doping treatment is not performed.

For example, in the case where gallium oxide whose composition isrepresented by GaO_(x)(x>0) is used, since a single crystal of galliumoxide is Ga₂O₃, x can be greater than 1.5 and less than 6 (i.e., theamount of O is greater than 1.5 times of that of Ga and less than 6times of that of Ga). Alternatively, for example, in the case wheresilicon oxide whose composition is represented by SiO_(x) (x>0) is used,when SiO₂ (i.e., the amount of O is double of that of Si) is employed, xis greater than 2 and less than 8 (i.e., the amount of O is greater thandouble of that of Si and less than 8 times of that of Si). Note thatsuch an oxygen excessive region may exist in part of the insulating film(including its interface).

As described above, when the oxygen excessive region is formed in thegate insulating film 110 or the vicinity of the interface between thegate insulating film 110 and the oxide semiconductor film 108, oxygencan be prevented from being transferred from the oxide semiconductorfilm 108 to the gate insulating film 110. Accordingly, when oxygendoping treatment is performed on the gate insulating film 110, anexcessive amount of oxygen can be contained in the oxide semiconductorfilm 108 as compared to hydrogen.

Further, in the oxygen doping treatment described above, oxygen can besupplied into the oxide semiconductor film 108, the vicinity of theinterface between the oxide semiconductor film 108 and the insulatingfilm 102, or the insulating film 102. In particular, oxygen is easilysupplied into the oxide semiconductor film 108, the vicinity of theinterface between the oxide semiconductor film 108 and the insulatingfilm 102, or the insulating film 102 in the case where the thickness ofthe gate insulating film 110 or the oxide semiconductor film 108 issufficiently small, in the case where an electrical bias applied to thesubstrate is sufficiently large, or in the case where heat treatment isperformed after oxygen doping treatment.

In addition, at least part of the oxygen 180 c added to the gateinsulating film 110 preferably has a dangling bond in the oxidesemiconductor film after being supplied to the oxide semiconductor. Thisis because, with the dangling bond, the oxygen 180 c can be bonded withhydrogen which remains in the film, so that the hydrogen can be fixed(made to be an immovable ion).

The oxygen 180 c described above can be generated by a plasma generatingapparatus or an ozone generating apparatus. More specifically, forexample, an apparatus for etching of a semiconductor device, anapparatus for ashing of a resist mask, or the like can be used togenerate the oxygen 180 c, and the gate insulating film 110 isprocessed.

Note that it is preferable to apply an electrical bias to the substratein order to perform oxygen doping more favorably.

Note that after the oxygen doping treatment, heat treatment ispreferably performed. By the heat treatment, an excessive amount ofoxygen can be supplied to the oxide semiconductor film as compared tohydrogen. In addition, the heat treatment may also serve as the aboveheat treatment for supply of oxygen. There is no limitation on thetiming of heat treatment for achieving the effect as long as it is afterthe oxygen doping treatment. Further, the oxygen doping treatment andthe heat treatment may be repeated. By repeatedly performing the oxygendoping treatment and the heat treatment, the transistor can have higherreliability. Note that the number of repetitions can be setappropriately.

Then, the gate electrode 112 is formed (see FIG. 5C). The gate electrode112 can be formed using a metal material such as molybdenum, titanium,tantalum, tungsten, aluminum, copper, neodymium, or scandium or an alloymaterial which contains any of these materials as its main component.Note that the gate electrode 112 may have a single-layer structure or alayered structure.

Note that an insulating film may be formed after formation of the gateelectrode 112. The insulating film may be formed using silicon oxide,silicon nitride, aluminum oxide, aluminum nitride, gallium oxide, amixed material of thereof, or the like. In particular, a silicon nitridefilm is preferable as the insulating film because added oxygen can beprevented from being released to the outside and hydrogen or the likefrom the outside can be effectively prevented from being entered intothe oxide semiconductor film 108. A wiring connected to the sourceelectrode 104 a, the drain electrode 104 b, the gate electrode 112, orthe like may be formed.

Through the above process, the transistor 120 is formed.

In that case, the amount of oxygen contained in the oxide semiconductorfilm 108 is greater than the stoichiometric proportion of the oxidesemiconductor film 108, preferably greater than the stoichiometricproportion and less than double of the stoichiometric proportion.Alternatively, the amount of oxygen may be greater than Y, preferablygreater than Y and less than 2Y, where the amount of oxygen in the casewhere the material of the oxide semiconductor film 108 is a singlecrystal is Y. Still alternatively, the amount of oxygen may be greaterthan Z, preferably greater than Z and less than 2Z based on the amountof oxygen Z in the oxide semiconductor film in the case where oxygendoping is not performed. The reason of the presence of the upper limitin the above preferable range is that the oxide semiconductor film 108might take hydrogen like a hydrogen storing alloy (a hydrogen storagealloy) when the amount of oxygen is too large

In the case of a material whose crystalline structure is represented byInGaO₃(ZnO)_(m) (m>0), x in InGaZnO_(x) can be greater than 4 and lessthan 8 when the crystalline structure where m is 1 (InGaZnO₄) is used asthe reference, and x in InGaZn₂O_(x) can be greater than 5 and less than10 when the crystalline structure where in is 2 (InGaZn₂O₅) is used asthe reference. Such an oxygen excessive region may exist in part of theoxide semiconductor (including the interface).

As described above, the oxygen excessive region is formed in the oxidesemiconductor film 108 and an excessive amount of oxygen can becontained in the oxide semiconductor film 108 as compared to hydrogen.

Note that the above description gives the example in which oxygen dopingtreatment is performed on all of the insulating film 102, the oxidesemiconductor film 108, and the gate insulating film 110; however, anembodiment of the disclosed invention is not limited thereto. Forexample, the oxygen doping treatment may be performed on the insulatingfilm 102 and the gate insulating film 110, or on the oxide semiconductorfilm 108 and the gate insulating film 110.

The transistor according to this embodiment includes a highly-purifiedand i-type (intrinsic) oxide semiconductor film which is obtained insuch a manner that an impurity including a hydrogen atom, such ashydrogen, water, a hydroxyl group, and hydride (also referred to as ahydrogen compound), is removed from an oxide semiconductor by heattreatment, and oxygen, which might be reduced in a step for removing animpurity, is supplied so that an excessive amount of oxygen is containedin the oxide semiconductor as compared to hydrogen. The transistorincluding the oxide semiconductor film which is highly purified in theabove manner has suppressed variation in the electrical characteristicssuch as a threshold voltage and is electrically stable.

Since the bonding strength between indium and oxygen is relatively weak,when an oxide semiconductor material containing indium is used as theoxide semiconductor film and the insulating film in contact with theoxide semiconductor film includes a material, such as silicon, whosebonding strength with oxygen is stronger, there is a possibility thatoxygen in the oxide semiconductor film is abstracted by heat treatmentso that oxygen deficiency is formed in the vicinity of the interface ofthe oxide semiconductor film. However, in a transistor according to anembodiment of the disclosed invention, oxygen deficiency due toabstraction of oxygen from the oxide semiconductor film can be preventedby supplying an excessive amount of oxygen to the insulating film incontact with the oxide semiconductor film.

In particular, when the gate insulating film 110 or the insulating film102 includes an oxygen excessive region by performing oxygen dopingtreatment on the gate insulating film 110 or the insulating film 102,oxygen can be prevented from being transferred from the oxidesemiconductor film 108 to the gate insulating film 110 or the insulatingfilm 102. In addition, by performing oxygen doping treatment on theoxide semiconductor film 108, the amount of oxygen contained in theoxide semiconductor film 108 can be increased. Accordingly, when theexcessive amount of oxygen is contained in the oxide semiconductor filmas compared to hydrogen, degradation due to electrical bias stress orthermal stress can be suppressed and degradation due to light can bereduced.

As described above, according to an embodiment of the disclosedinvention, a transistor having excellent reliability can be provided.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

Embodiment 3

In this embodiment, another example of a method for manufacturing asemiconductor device will be described with reference to FIGS. 6A to 6F.

<Structural Example of Semiconductor Device>

The structure of a semiconductor device manufactured in accordance witha method for manufacturing a semiconductor device of this embodiment isthe same as that of the transistor 120 of the above embodiment. In otherwords, the semiconductor device includes, over the substrate 100, theinsulating film 102, the source electrode 104 a, the drain electrode 104b, the oxide semiconductor film 108, the gate insulating film 110, andthe gate electrode 112 (see FIGS. 1 to 1C).

As described in the above embodiment, the gate insulating film 110 inthe transistor 120 is an insulating film subjected to oxygen dopingtreatment and includes an oxygen excessive region. When the gateinsulating film 110 includes the oxygen excessive region, oxygen can beprevented from being transferred from the oxide semiconductor film 108to the gate insulating film 110. In addition, oxygen can be suppliedfrom the gate insulating film 110 to the oxide semiconductor film 108.Further, in this embodiment, oxygen doping treatment can be directlyperformed on the oxide semiconductor film 108, and oxygen dopingtreatment can also be performed on the insulating film 102. By suchoxygen doping treatment, an excessive amount of oxygen can be containedin the oxide semiconductor film 108 as compared to hydrogen.Accordingly, the transistor 120 which further increases its reliabilitycan be obtained. In addition, the oxygen doping treatment performed onthe insulating film 102 in this embodiment also serves as a step forremoving a mask 103 a and a mask 103 b used for foaming the sourceelectrode 104 a and the drain electrode 104 b. By employing such aprocess, manufacturing cost can be reduced owing to simplification ofsteps. Note that in a similar to the above embodiment, transistorshaving different structures can also be manufactured (see FIGS. 3A to3D).

<Example of Manufacturing Process of Semiconductor Device>

An example of steps for manufacturing the semiconductor device will bedescribed below with reference to FIGS. 6A to 6F. Note that the basiccontents of the manufacturing steps are substantially the same as thoseof the above embodiments; therefore, only different points will bedescribed below.

First, the insulating film 102 is formed over the substrate 100 (seeFIG. 6A). The description of FIG. 4A can be referred to for the detailsthereof.

Next, a conductive film for forming the source electrode and the drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over the insulating film102 and the conductive film is processed with the use of the mask 103 aand the mask 103 b, thereby forming the source electrode 104 a and thedrain electrode 104 b. Then, treatment using oxygen 180 a (also referredto as oxygen doping treatment or oxygen plasma doping treatment) isperformed on the insulating film 102 (see FIG. 6B). The description ofFIG. 4C can be referred to for the details of the steps for forming thesource electrode 104 a and the drain electrode 104 b. Here, the oxygendoping treatment also serves as the step for removing the mask 103 a andthe mask 103 b.

The oxygen 180 a contains at least any of an oxygen radical, an oxygenatom, and an oxygen ion. By performing oxygen doping treatment on theinsulating film 102, oxygen can be contained in the insulating film 102and either or both in the oxide semiconductor film 108 formed lateror/and in the vicinity of the interface of the oxide semiconductor film108. In that case, the amount of oxygen contained in the insulating film102 is greater than the stoichiometric proportion of the insulating film102, or is preferably greater than the stoichiometric proportion andless than four times of the stoichiometric proportion, more preferablygreater than the stoichiometric proportion and less than double of thestoichiometric proportion. Alternatively, the amount of oxygen containedin the insulating film 102 can be greater than Y, or can be preferablygreater than Y and less than 4Y, where the amount of oxygen in the casewhere the material of the insulating film 102 is a single crystal is Y.Still alternatively, the amount of oxygen contained in the insulatingfilm 102 can be greater than Z, and can be preferably greater than Z orless than 4Z based on the amount of oxygen Z in the insulating film inthe case where oxygen doping treatment is not performed.

For example, in the case where gallium oxide whose composition isrepresented by GaO_(x) (x>0) is used, since a single crystal of galliumoxide is Ga₂O₃, x can be greater than 1.5 and less than 6 (i.e., theamount of O is greater than 1.5 times of that of Ga and less than 6times of that of Ga). Alternatively, for example, in the case wheresilicon oxide whose composition is represented by SiO_(x) (x>0) is used,when SiO₂ (i.e., the amount of O is double of that of Si) is employed, xis greater than 2 and less than 8 (i.e., the amount of O is greater thandouble of that of Si and less than 8 times of that of Si). Note thatsuch an oxygen excessive region may exist in part of the insulating film(including its interface).

As described above, when the oxygen excessive region is formed in theinsulating film 102 or the vicinity of the interface between theinsulating film 102 and the oxide semiconductor film 108 formed later,oxygen can be prevented from being transferred from the oxidesemiconductor film 108 to the insulating film 102. Accordingly, whenoxygen doping treatment is performed on the insulating film 102, anexcessive amount of oxygen can be contained in the oxide semiconductorfilm 108 as compared to hydrogen.

In addition, at least part of the oxygen 180 a added to the insulatingfilm preferably has a dangling bond in the oxide semiconductor filmafter being supplied to the oxide semiconductor. This is because, withthe dangling bond, the oxygen 180 a can be bonded with hydrogen whichremains in the film, so that the hydrogen can be fixed (made to be animmovable ion).

The oxygen 180 a can be generated by a plasma generating apparatus or anozone generating apparatus. Specifically, for example, the oxygen 180 ais generated with the use of an apparatus for ashing of a resist mask orthe like, and the insulating film 102 can be processed.

By the oxygen doping treatment, the mask 103 a and the mask 103 b areremoved. Note that, unlike a general step for removing a mask, the stepis performed to add oxygen; therefore, it is preferable that arelatively-strong bias be applied to the substrate.

In addition, by the oxygen doping treatment, a region containing oxygenat high concentration and a region containing oxygen at lowconcentration are formed in the insulating film 102. Specifically, inthe insulating film 102, a region which is not covered with the sourceelectrode 104 a and the drain electrode 104 b is the region containingoxygen at high concentration, and a region which is covered with thesource electrode 104 a and the drain electrode 104 b is the regioncontaining oxygen at low concentration.

Next, an oxide semiconductor film in contact with the source electrode104 a and the drain electrode 104 b is formed over the insulating film102 and the oxide semiconductor film is processed, so that anisland-shaped oxide semiconductor film is formed. Then, heat treatmentis performed on the island-shaped oxide semiconductor film, whereby thehighly-purified oxide semiconductor film 108 is formed (see FIG. 6C).The description of FIGS. 4D and 4E can be referred to for the details ofthe steps.

Then, the treatment using oxygen 180 b is performed on the oxidesemiconductor film 108 (see FIG. 6D). The description of FIG. 4F can bereferred to for the details thereof.

Next, the gate insulating film 110 which is in contact with part of theoxide semiconductor film 108 and covers the source electrode 104 a andthe drain electrode 104 b is formed. After that, treatment using oxygen180 c is performed on the gate insulating film 110 (see FIG. 6E). Thedescription of FIGS. 5A and 5B may be referred to for the detailsthereof.

Then, the gate electrode 112 is formed (see FIG. 6F). The description ofFIG. 5C can be referred to for the details thereof.

Note that after the gate electrode 112 is formed, an insulating film maybe formed. The insulating film can be formed using silicon oxide,silicon nitride, aluminum oxide, aluminum nitride, gallium oxide, or amixed material thereof, for example. In particular, it is preferablethat a silicon nitride film be used as the insulating film because addedoxygen can be prevented from being released to the outside, and hydrogenor the like from the outside can be effectively prevented from enteringthe oxide semiconductor film 108. In addition, a wiring connected to thesource electrode 104 a, the drain electrode 104 b, or the gate electrode112 may be formed.

Through the above process, the transistor 120 is formed.

Note that the above description gives the example in which oxygen dopingtreatment is performed on all of the insulating film 102, the oxidesemiconductor film 108, and the gate insulating film 110; however, anembodiment of the disclosed invention is not limited thereto. Forexample, the oxygen doping treatment may be performed on the insulatingfilm 102 and the gate insulating film 110.

The transistor according to this embodiment includes a highly-purifiedand i-type (intrinsic) oxide semiconductor film which is obtained insuch a manner that an impurity including a hydrogen atom, such ashydrogen, water, a hydroxyl group, and hydride (also referred to as ahydrogen compound), is removed from an oxide semiconductor by heattreatment, and oxygen, which might be reduced in a step for removing animpurity, is supplied so that an excessive amount of oxygen is containedin the oxide semiconductor as compared to hydrogen. The transistorincluding the oxide semiconductor film which is highly purified in theabove manner has suppressed variation in the electrical characteristicssuch as a threshold voltage and is electrically stable.

Since the bonding strength between indium and oxygen is relatively weak,when an oxide semiconductor material containing indium is used as theoxide semiconductor film and the insulating film in contact with theoxide semiconductor film includes a material whose bonding strength withoxygen is stronger, such as silicon, there is a possibility that oxygenin the oxide semiconductor film is abstracted by heat treatment so thatoxygen deficiency is formed in the vicinity of the interface of theoxide semiconductor film. However, in a transistor according to anembodiment of the disclosed invention, oxygen deficiency due toabstraction of oxygen from the oxide semiconductor film can be preventedby supplying an excessive amount of oxygen to the insulating film incontact with the oxide semiconductor film.

In particular, when the gate insulating film 110 or the insulating film102 includes an oxygen excessive region by performing oxygen dopingtreatment on the gate insulating film 110 or the insulating film 102,oxygen can be prevented from being transferred from the oxidesemiconductor film 108 to the gate insulating film 110 or the insulatingfilm 102. In addition, by performing oxygen doping treatment on theoxide semiconductor film 108, the amount of oxygen contained in theoxide semiconductor film 108 can be increased. Accordingly, when theexcessive amount of oxygen is contained in the oxide semiconductor filmas compared to hydrogen, degradation due to electrical bias stress orthermal stress can be suppressed and degradation due to light can bereduced.

In addition, according to the manufacturing method of this embodiment,the process is simplified and therefore, cost for manufacture can bereduced.

As described above, according to an embodiment of the disclosedinvention, a transistor having excellent reliability can be providedwhile manufacturing cost is reduced.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

Embodiment 4

In this embodiment, an example of a plasma apparatus (also referred toas an ashing apparatus) which can be used for oxygen doping treatmentwill be described. Note that the apparatus is industrially suitable ascompared to an ion implantation apparatus or the like because theapparatus can be applicable for a large-sized glass substrate of thefifth generation or later, for example.

FIG. 17A illustrates an example of a top view of a single wafermulti-chamber equipment. FIG. 17B illustrates an example of across-sectional view of a plasma apparatus (also referred to as anashing apparatus) used for oxygen plasma doping.

The single wafer multi-chamber equipment illustrated in FIG. 17Aincludes three plasma apparatuses 10 each of which corresponds to FIG.17B, a substrate supply chamber 11 including three cassette ports 14 forholding a process substrate, a load lock chamber 12, a transfer chamber13, and the like. A substrate supplied to the substrate supply chamberis transferred through the load lock chamber 12 and the transfer chamber13 to a vacuum chamber 15 in the plasma apparatus 10 and is subjected tooxygen plasma doping. The substrate which has been subjected to oxygenplasma doping is transferred from the plasma apparatus 10, through theload lock chamber 12 and the transfer chamber 13, to the substratesupply chamber 11. Note that a transfer robot for transferring a processsubstrate is provided in each of the substrate supply chamber 11 and thetransfer chamber 13.

Referring to FIG. 17B, the plasma apparatus 10 includes the vacuumchamber 15. A plurality of gas outlets and an ICP coil (an inductivelycoupled plasma coil) 16 which is a generation source of plasma areprovided on a top portion of the vacuum chamber 15.

The twelve gas outlets are arranged in a center portion, seen from thetop of the plasma apparatus 10. Each of the gas outlets is connected toa gas supply source for supplying an oxygen gas, through a gas flow path17. The gas supply source includes a mass flow controller and the likeand can supply an oxygen gas to the gas flow path 17 at a desired flowrate (which is greater than 0 sccm and less than or equal to 1000 sccm).The oxygen gas supplied from the gas supply source is supplied from thegas flow path 17, through the twelve gas outlets, into the vacuumchamber 15.

The ICP coil 16 includes a plurality of strip-like conductors each ofwhich has a spiral form. One end of each of the conductors iselectrically connected to a first high-frequency power source 18 (13.56MHz) through a matching circuit for controlling impedance, and the otherend thereof is grounded.

A substrate stage 19 functioning as a bottom electrode is provided in alower portion of the vacuum chamber. By an electrostatic chuck or thelike provided for the substrate stage 19, a process substrate 20 is heldon the substrate stage so as to be detachable. The substrate stage 19 isprovided with a heater as a heating system and a He gas flow pass as acooling system. The substrate stage is connected to a secondhigh-frequency power source 21 (3.2 MHz) for applying a substrate biasvoltage.

In addition, the vacuum chamber 15 is provided with an exhaust port andan automatic pressure control valve (also referred to as an APC) 22. TheAPC is connected to a turbo molecular pump 23 and further, connected toa dry pump 24 through the turbo molecular pump 23. The APC controls theinside pressure of the vacuum chamber. The turbo molecular pump 23 andthe dry pump 24 reduce the inside pressure of the vacuum chamber 15.

Next, described is an example in which plasma is generated in the vacuumchamber 15 illustrated in FIG. 17B, and oxygen plasma doping isperformed on an oxide semiconductor film, a base insulating film, or agate insulating film provided for the process substrate 20.

First, the inside pressure of the vacuum chamber 15 is held at a desiredpressure by operating the turbo molecular pump 23, the dry pump 24, andthe like, and then, the process substrate 20 is installed on thesubstrate stage in the vacuum chamber 15. Note that the processsubstrate 20 held on the substrate stage has at least an oxidesemiconductor film or a base insulating film. In this embodiment, theinside pressure of the vacuum chamber 15 is held at 1.33 Pa. Note thatthe flow rate of the oxygen gas supplied from the gas outlets into thevacuum chamber 15 is set at 250 sccm.

Next, a high-frequency power is applied from the first high-frequencypower source 18 to the ICP coil 16, thereby generating plasma. Then, astate in which plasma is being generated is kept for a certain period(greater than or equal to 30 seconds and less than or equal to 600seconds). Note that the high-frequency power applied to the ICP coil 16is greater than or equal to 1 kW and less than or equal to 10 kW. Inthis embodiment, the high-frequency power is set at 6000 W. At thistime, a substrate bias voltage may be applied from the secondhigh-frequency power source 21 to the substrate stage. In thisembodiment, the power used for applying the substrate bias voltage isset at 1000 W.

In this embodiment, the state in which plasma is being generated is keptfor 60 seconds and then, the process substrate 20 is transferred fromthe vacuum chamber 15. In this manner, oxygen plasma doping can beperformed on the oxide semiconductor film, the base insulating film, orthe gate insulating film provided for the process substrate 20.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

Embodiment 5

In this embodiment, as an example of a semiconductor device, a memorymedium (a memory element) will be described. In this embodiment, thetransistor including an oxide semiconductor described in any ofEmbodiments 1 to 3 or the like and a transistor including a materialother than an oxide semiconductor are formed over one substrate.

FIGS. 7A to 7C illustrate an example of a structure of a semiconductordevice. FIG. 7A illustrates a cross section of the semiconductor device,and FIG. 7B illustrates a plan view of the semiconductor device. Here,FIG. 7A corresponds to a cross section taken along lines C1-C2 and D1-D2of FIG. 7B. FIG. 7C illustrates an example of a diagram of a circuitincluding the semiconductor device as a memory element. In thesemiconductor device illustrated in FIGS. 7A and 7B, a transistor 240including a first semiconductor material is provided in a lower portion,and the transistor 120 described in Embodiment 1 is provided in an upperportion. Note that the transistor 120 includes an oxide semiconductor asa second semiconductor material. In this embodiment, the firstsemiconductor material is a semiconductor material other than an oxidesemiconductor. As the semiconductor material other than an oxidesemiconductor, for example, silicon, germanium, silicon germanium,silicon carbide, gallium arsenide, or the like can be used, and a singlecrystal semiconductor is preferably used. Alternatively, an organicsemiconductor material or the like may be used. A transistor includingsuch a semiconductor material other than an oxide semiconductor caneasily achieve high-speed operation. On the other hand, a transistorincluding an oxide semiconductor can hold charge for a long time owingto its characteristics.

Note that in this embodiment, an example in which the memory medium isformed using the transistor 120 is described; however, needless to say,any of the transistor 130, the transistor 140, the transistor 150, thetransistor 160, and the like described in Embodiment 1 or 2 can be usedinstead of the transistor 120.

The transistor 240 in FIGS. 7A to 7C includes a channel formation region216 provided in a substrate 200 including a semiconductor material(e.g., silicon), impurity regions 220 between which the channelformation region 216 is provided, metal compound regions 224 in contactwith the impurity regions 220, a gate insulating film 208 provided overthe channel formation region 216, and the gate electrode 210 providedover the gate insulating film 208.

As the substrate 200 including a semiconductor material, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate of silicon, silicon carbide, or the like; a compoundsemiconductor substrate of silicon germanium or the like; an SOIsubstrate; or the like can be used. Note that although the term “SOIsubstrate” generally means a substrate in which a silicon semiconductorfilm is provided over an insulating surface, the term “SOI substrate” inthis specification and the like also includes a substrate in which asemiconductor film including a material other than silicon is providedover an insulating surface. In other words, a semiconductor filmincluded in the “SOI substrate” is not limited to a siliconsemiconductor film. Moreover, the SOI substrate can be a substratehaving a structure in which a semiconductor film is provided over aninsulating substrate such as a glass substrate with an insulating filmprovided therebetween.

An element isolation insulating film 206 is provided over the substrate200 so as to surround the transistor 240, and an insulating film 228 andan insulating film 230 are provided to cover the transistor 240. Notethat for high integration, it is preferable that, as in FIG. 7A, thetransistor 240 does not have a sidewall insulating film On the otherhand, in the case where the characteristics of the transistor 240 havepriority, sidewall insulating films may be provided on side surfaces ofthe gate electrode 210, and the impurity regions 220 may each include aregion with a different impurity concentration.

The transistor 240 can be manufactured using silicon, germanium, silicongermanium, silicon carbide, gallium arsenide, or the like. Such atransistor 240 is capable of high speed operation. Thus, when thetransistor is used as a reading transistor, data can be read out at highspeed.

After the transistor 240 is formed, as treatment prior to the formationof the transistor 120 and a capacitor 164, the insulating film 228 andthe insulating film 230 are subjected to CMP treatment so that a topsurface of the gate electrode 210 is exposed. As treatment for exposingthe top surface of the gate electrode 210, etching treatment or the likecan also be employed instead of CMP treatment; in order to improvecharacteristics of the transistor 120, surfaces of the insulating film228 and the insulating film 230 are preferably made as flat as possible.

Next, a conductive film is formed over the gate electrode 210, theinsulating film 228, the insulating film 230, and the like and theconductive film is selectively etched, so that a source electrode 104 aand a drain electrode 104 b are formed.

The conductive film can be formed by a PVD method such as a sputteringmethod or a CVD method such as a plasma CVD method. As the material ofthe conductive film, an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W, an alloy including any of the above elements as its component, orthe like can be used. Any of Mn, Mg, Zr, Be, Nd, and Sc, or a materialincluding any of these in combination may be used.

The conductive film may have either a single-layer structure or alayered structure of two or more layers. For example, the conductivefilm can have a single-layer structure of a titanium film or a titaniumnitride film, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, or a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order. Note that in the case where the conductive film has asingle-layer structure of a titanium film or a titanium nitride film,there is an advantage that the source electrode 104 a and the drainelectrode 104 b can be easily processed to be tapered.

A channel length (L) of the transistor 120 in the upper portion isdetermined by a distance between a lower end portion of the sourceelectrode 104 a and a lower end portion of the drain electrode 104 b.Note that for light exposure for forming a mask used in the case where atransistor with a channel length (L) of less than 25 nm is formed, it ispreferable to use extreme ultraviolet rays whose wavelength is as shortas several nanometers to several tens of nanometers.

Next, an oxide semiconductor film is formed to cover the sourceelectrode 104 a and the drain electrode 104 b, and the oxidesemiconductor film is selectively etched, so that the oxidesemiconductor film 108 is formed. The oxide semiconductor film is formedusing the material and the formation process described in Embodiment 1.

Then, a gate insulating film 110 in contact with the oxide semiconductorfilm 108 is formed. The gate insulating film 110 is formed using thematerial and the formation process described in Embodiment 1.

Next, over the gate insulating film 110, a gate electrode 112 a isformed in a region overlapping with the oxide semiconductor film 108 andan electrode 112 b is formed in a region overlapping with the sourceelectrode 104 a.

After the gate insulating film 110 is formed, heat treatment (alsoreferred to as supply of oxygen) is preferably performed in an inert gasatmosphere or an oxygen atmosphere. The temperature of the heattreatment is higher than or equal to 200° C. and lower than or equal to450° C., preferably higher than or equal to 250° C. and lower than orequal to 350° C. For example, the heat treatment may be performed at250° C. for one hour in a nitrogen atmosphere. By performing the heattreatment, variation in electrical characteristics of the transistor canbe reduced.

Note that the timing of the heat treatment for supplying oxygen is notlimited thereto. For example, the heat treatment for supplying oxygenmay be performed after the gate electrode is formed. Alternatively, heattreatment for supply of oxygen may be performed following heat treatmentfor dehydration or the like; heat treatment for dehydration or the likemay also serve as heat treatment for supplying oxygen; or heat treatmentfor supplying oxygen may also serve as heat treatment for dehydration orthe like.

As described above, when heat treatment for dehydration or the like, andoxygen doping or heat treatment for supplying oxygen are performed, theoxide semiconductor film 108 can be highly purified so as to containimpurities as little as possible.

The gate electrode 112 a and the electrode 112 b can be formed in such amanner that a conductive film is formed over the gate insulating film110 and then etched selectively.

Next, an insulating film 151 and an insulating film 152 are formed overthe gate insulating film 110, the gate electrode 112 a, and theelectrode 112 b. The insulating film 151 and the insulating film 152 canbe formed by a sputtering method, a CVD method, or the like. Theinsulating film 151 and the insulating film 152 can be formed using amaterial including an inorganic insulating material such as siliconoxide, silicon oxynitride, silicon nitride, hafnium oxide, aluminumoxide, or gallium oxide.

Next, an opening reaching the drain electrode 104 b is formed throughthe gate insulating film 110, the insulating film 151, and theinsulating film 152. The opening is formed by selective etching with theuse of a mask or the like.

After that, an electrode 154 is formed in the opening, and a wiring 156which is in contact with the electrode 154 is formed over the insulatingfilm 152.

The electrode 154 can be formed in such a manner, for example, that aconductive film is formed in a region including the opening by a PVDmethod, a CVD method, or the like and then part of the conductive filmis removed by etching, CMP, or the like.

The wiring 156 is formed in such a manner that a conductive film isformed by a PVD method such as a sputtering method or a CVD method suchas a plasma CVD method, and then the conductive film is patterned. As amaterial of the conductive film, an element selected from Al, Cr, Cu,Ta, Ti, Mo, and W, an alloy including any of the above elements as itscomponent, or the like can be used. Any of Mn, Mg, Zr, Be, Nd, and Sc,or a material including any of these in combination may be used. Thedetails are the same as those of the source electrode 104 a, the drainelectrode 104 b, or the like.

Through the above process, the transistor 120 and the capacitor 164including the highly-purified oxide semiconductor film 108 arecompleted. The capacitor 164 includes the source electrode 104 a, theoxide semiconductor film 108, the gate insulating film 110, and theelectrode 112 b.

Note that in the capacitor 164 in FIGS. 7A to 7C, insulation between thesource electrode 104 a and the electrode 112 b can be sufficientlysecured by stacking the oxide semiconductor film 108 and the gateinsulating film 110. Needless to say, the capacitor 164 without theoxide semiconductor film 108 may be employed in order to securesufficient capacitance. Further alternatively, the capacitor 164 may beomitted in the case where a capacitor is not needed.

FIG. 7C illustrates an example of a diagram of a circuit using thesemiconductor device as a memory element. In FIG. 7C, one of a sourceelectrode and a drain electrode of the transistor 120, one electrode ofthe capacitor 164, and a gate electrode of the transistor 240 areelectrically connected to each other. A first wiring (1st Line, alsoreferred to as a source line) is electrically connected to a sourceelectrode of the transistor 240. A second wiring (2nd Line, alsoreferred to as a bit line) is electrically connected to a drainelectrode of the transistor 240. A third wiring (3rd Line, also referredto as a first signal line) is electrically connected to the other of thesource electrode and the drain electrode of the transistor 120. A fourthwiring (4th Line, also referred to as a second signal line) iselectrically connected to a gate electrode of the transistor 120. Afifth wiring (5th Line, also referred to as a word line) is electricallyconnected to the other electrode of the capacitor 164.

The transistor 120 including an oxide semiconductor has extremely lowoff current; therefore, when the transistor 120 is turned off, thepotential of a node (hereinafter, a node FG) where one of the sourceelectrode and drain electrode of the transistor 120, one electrode ofthe capacitor 164, and the gate electrode of the transistor 240 areelectrically connected to each other can be held for an extremely longtime. The capacitor 164 facilitates holding of charge given to the nodeFG and reading of the held data.

When data is stored in the semiconductor device (writing), the potentialof the fourth wiring is set to a potential at which the transistor 120is turned on, whereby the transistor 120 is turned on. Thus, thepotential of the third wiring is applied to the node FG and apredetermined amount of charge is accumulated in the node FG Here,charge for applying either of two different potential levels(hereinafter referred to as a low-level charge and a high-level charge)is given to the node FG After that, the potential of the fourth wiringis set to a potential at which the transistor 120 is turned off, wherebythe transistor 120 is turned off. This makes the node FG floating andthe predetermined amount of charge is held in the node FG. Thepredetermined amount of charge is thus accumulated and held in the nodeFG whereby the memory cell can store data.

Since the off current of the transistor 120 is extremely small, thecharge applied to the node FG is held for a long time. This further canremove the need of refresh operation or drastically reduce the frequencyof the refresh operation, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be held for a long time even whenpower is not supplied.

When stored data is read out (reading), while a predetermined potential(a fixed potential) is applied to the first wiring, an appropriatepotential (a read-out potential) is applied to the fifth wiring, wherebythe transistor 240 changes its state depending on the amount of chargeheld in the node FG This is because, in general, when the transistor 240is an n-channel transistor, an apparent threshold value V_(th) _(—) _(H)of the transistor 240 in the case where a high-level charge is held inthe node FG is lower than an apparent threshold value V_(th) _(—) _(L)of the transistor 240 in the case where a low-level charge is held inthe node FG. Here, an apparent threshold voltage refers to the potentialof the fifth wiring, which is needed to turn on the transistor 240.Thus, by setting the potential of the fifth wiring to a potential V₀which is between V_(th) _(—) _(H) and V_(th) _(—) _(L), charge held inthe node FG can be determined. For example, in the case where ahigh-level charge is given in writing, when the potential of the fifthwiring is set to V₀ (>V_(th) _(—) _(H)), the transistor 240 is turnedon. In the case where a low-level charge is given in writing, even whenthe potential of the fifth wiring is set to V₀ (<V_(th) _(—) _(L)), thetransistor 240 remains in an off state. In such a manner, by controllingthe potential of the fifth wiring and determining whether the transistor240 is in an on state or off state (reading out the potential of thesecond wiring), stored data can be read out.

In order to rewrite stored data, a new potential is applied to the nodeFG that is holding the predetermined amount of charge given in the abovewriting, so that the charge of the new data is held in the node FGSpecifically, the potential of the fourth wiring is set to a potentialat which the transistor 120 is turned on, whereby the transistor 120 isturned on. Consequently, the potential of the third wiring (a potentialof new data) is applied to the node FG, and the predetermined amount ofcharge is accumulated in the node FG After that, the potential of thefourth wiring is set to a potential at which the transistor 120 isturned off, whereby the transistor 120 is turned off. Thus, charge ofthe new data is held in the node FG In other words, while thepredetermined amount of charge given in the first writing is held in thenode FG the same operation (a second writing) as that in the firstwriting is performed, whereby the stored data can be overwritten.

The off current of the transistor 120 described in this embodiment canbe sufficiently reduced by using the highly-purified, and intrinsicoxide semiconductor film 108. In addition, the oxide semiconductor film108 contains excessive oxygen, whereby variation in the electricalcharacteristics of the transistor 120 is suppressed, so that thetransistor which is electrically stable can be obtained. Further, withthe use of such a transistor, a highly-reliable semiconductor devicecapable of holding stored data for an extremely long time can beobtained.

In the semiconductor device described in this embodiment, the transistor240 and the transistor 120 overlap with each other; therefore, asemiconductor device whose integration degree is sufficiently improvedcan be realized.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

Embodiment 6

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using the transistor exemplified inany of Embodiments 1 to 3. Moreover, part or all of the driver circuitrywhich includes the transistor can be formed over a substrate where apixel portion is formed, whereby a system-on-panel can be obtained.

In FIG. 8A, a sealant 4005 is provided so as to surround a pixel portion4002 provided over a first substrate 4001, and the pixel portion 4002 issealed by using a second substrate 4006. In FIG. 8A, a signal linedriver circuit 4003 and a scan line driver circuit 4004 which areseparately prepared on a substrate using a single crystal semiconductorfilm or a polycrystalline semiconductor film are mounted in a regionthat is different from the region surrounded by the sealant 4005 overthe first substrate 4001. Various signals and potentials are supplied tothe signal line driver circuit 4003 and the scan line driver circuit4004 which are separately formed and to the pixel portion 4002 fromflexible printed circuits (FPCs) 4018 a and 4018 b.

In FIGS. 8B and 8C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Consequently, the pixel portion 4002 and the scan line drivercircuit 4004 are sealed together with the display element, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 8B and 8C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate prepared separately is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. In FIGS. 8B and 8C, various signals andpotential are supplied to the signal line driver circuit 4003 which isseparately formed, the scan line driver circuit 4004, and the pixelportion 4002 from an FPC 4018.

An embodiment of the present invention is not limited to the structuresdescribed in FIGS. 8A to 8C. Only part of the signal line driver circuitor part of the scan line driver circuit may be separately formed andthen mounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method or the like can beused. FIG. 8A illustrates an example in which the signal line drivercircuit 4003 and the scan line driver circuit 4004 are mounted by a COGmethod. FIG. 8B illustrates an example in which the signal line drivercircuit 4003 is mounted by a COG method. FIG. 8C illustrates an examplein which the signal line driver circuit 4003 is mounted by a TAB method.

Note that the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

Note that the display device in this specification means an imagedisplay device, a display device, or a light source (including alighting device). Furthermore, the display device also includes thefollowing modules in its category: a module to which a connector such asan FPC, a TAB tape, or a TCP is attached; a module having a TAB tape ora TCP at the tip of which a printed wiring board is provided; and amodule in which an integrated circuit (IC) is directly mounted on adisplay element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors, and any of thetransistors which are described in Embodiments 1 to 3 can be appliedthereto.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by a current or avoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

An embodiment of the semiconductor device is described with reference toFIG. 9, FIG. 10, and FIG. 11. FIG. 9, FIG. 10, and FIG. 11 correspond tocross-sectional views taken along line M-N in FIG. 8B.

As illustrated in FIG. 9 to FIG. 11, the semiconductor device includes aconnection terminal electrode 4015 and a terminal electrode 4016. Theconnection terminal electrode 4015 and the terminal electrode 4016 areelectrically connected to a terminal included in the FPC 4018 through ananisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode 4016 is formed using the same conductive film as source anddrain electrodes of a transistor 4010 and a transistor 4011.

The pixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001 include a plurality oftransistors. In FIG. 9 to FIG. 11, the transistor 4010 included in thepixel portion 4002 and the transistor 4011 included in the scan linedriver circuit 4004 are illustrated as an example. In FIG. 10 and FIG.11, an insulating layer 4021 is provided over the transistor 4010 andthe transistor 4011.

In this embodiment, the transistor described in any of Embodiments 1 to3 can be applied to the transistor 4010 and the transistor 4011.Variation in electrical characteristics of the transistor 4010 and thetransistor 4011 is suppressed and the transistor 4010 and the transistor4011 are electrically stable. Therefore, highly-reliable semiconductordevices can be provided as the semiconductor devices illustrated in FIG.9 to FIG. 11.

The transistor 4010 provided in the pixel portion 4002 is electricallyconnected to a display element in a display panel. A variety of displayelements can be used as the display element as long as display can beperformed.

Note that an example of a liquid crystal display device using a liquidcrystal element as a display element is described in FIG. 9. In FIG. 9,a liquid crystal element 4013 which is a display element includes thefirst electrode layer 4030, the second electrode layer 4031, and aliquid crystal layer 4008. An insulating film 4032 and an insulatingfilm 4033 which serve as alignment films are provided so that the liquidcrystal layer 4008 is provided therebetween. The second electrode layer4031 is provided on the second substrate 4006 side, and the firstelectrode layer 4030 and the second electrode layer 4031 are stackedwith the liquid crystal layer 4008 provided therebetween.

A spacer 4035 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the thickness (acell gap) of the liquid crystal layer 4008. Note that the spacer is notlimited to a columnar spacer, and, for example, a spherical spacer maybe used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which appears just before a cholesteric phase changesinto an isotropic phase while temperature of cholesteric liquid crystalis increased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition which includes a liquid crystal exhibiting a blue phase anda chiral agent has a short response time of 1 msec or less, has opticalisotropy, which makes the alignment process unneeded, and has a smallviewing angle dependence. In addition, since an alignment film need notbe provided and rubbing treatment is unnecessary, electrostaticdischarge damage caused by the rubbing treatment can be prevented anddefects and damage of the liquid crystal display device can be reducedin the manufacturing process. Thus, productivity of the liquid crystaldisplay device can be increased.

The specific resistivity of the liquid crystal material is 1×10⁹ Ω·cm ormore, preferably 1×10¹¹ Ω·cm or more, more preferably 1×10¹² Ω·cm ormore. The value of the specific resistivity in this specification ismeasured at 20° C.

The size of a storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. By using the transistor including thehighly-purified oxide semiconductor film, it is enough to provide astorage capacitor having a capacitance that is ⅓ or less, preferably ⅕or less of a liquid crystal capacitance of each pixel.

In the transistor used in this embodiment, which includes thehighly-purified oxide semiconductor film, the current in an off state(the off current) can be made small. Accordingly, an electrical signalsuch as an image signal can be held for a long period, and a writinginterval can be set long in a state where power is being supplied.Accordingly, the frequency of refresh operation can be reduced, whichleads to an effect of suppressing power consumption.

In addition, the transistor including the highly-purified oxidesemiconductor film used in this embodiment can have relatively highfield-effect mobility and thus is capable of high speed operation.Therefore, by using the transistor in the pixel portion of the liquidcrystal display device, a high-quality image can be provided. Inaddition, since the transistors can be separately provided in a drivercircuit portion and a pixel portion over one substrate, the number ofcomponents of the liquid crystal display device can be reduced.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modemay be used. The vertical alignment mode is a method of controllingalignment of liquid crystal molecules of a liquid crystal display panel,in which liquid crystal molecules are aligned vertically to a panelsurface when no voltage is applied. Some examples are given as thevertical alignment mode. For example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an advanced superview (ASV) mode, or the like can be used. Moreover, it is possible touse a method called domain multiplication or multi-domain design, inwhich a pixel is divided into some regions (subpixels) and molecules arealigned in different directions in their respective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

In addition, it is possible to employ a time-division display method(also called a field-sequential driving method) with the use of aplurality of light-emitting diodes (LEDs) as a backlight. By employing afield-sequential driving method, color display can be performed withoutusing a color filter.

As a display method in the pixel portion, a progressive method, aninterlace method or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors of R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, the following can be used: R, G, B,and W (W corresponds to white); or R, G, B, and one or more of yellow,cyan, magenta, and the like. Further, the sizes of display regions maybe different between respective dots of the color elements. The presentinvention is not limited to the application to a display device forcolor display but can also be applied to a display device for monochromedisplay.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are injected from a pair of electrodes intoa layer containing a light-emitting organic compound, and current flows.The carriers (electrons and holes) are recombined, and thus, thelight-emitting organic compound is excited. The light-emitting organiccompound returns to a ground state from the excited state, therebyemitting light. Owing to such a mechanism, this light-emitting elementis referred to as a current-excitation light-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

In order to extract light emitted from the light-emitting element, atleast one of a pair of electrodes is transparent. The transistor and thelight-emitting element are provided over the substrate. Thelight-emitting element can have any of the following structures: a topemission structure in which light emission is extracted through thesurface opposite to the substrate; a bottom emission structure in whichlight emission is extracted through the surface on the substrate side;and a dual emission structure in which light emission is extractedthrough the surface opposite to the substrate and the surface on thesubstrate side.

An example of a light-emitting device in which a light-emitting elementis used as a display element is illustrated in FIG. 10. A light-emittingelement 4513 which is a display element is electrically connected to thetransistor 4010 provided in the pixel portion 4002. A structure of thelight-emitting element 4513 is not limited to the stacked-layerstructure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031,which is illustrated in FIG. 10. The structure of the light-emittingelement 4513 can be changed as appropriate depending on a direction inwhich light is extracted from the light-emitting element 4513, or thelike.

A partition wall 4510 is formed using an organic insulating material oran inorganic insulating material. It is particularly preferable that thepartition wall 4510 be formed using a photosensitive resin material tohave an opening over the first electrode layer 4030 so that a sidewallof the opening is formed as a tilted surface with continuous curvature.

The electroluminescent layer 4511 may be formed using a single layer ora plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe partition wall 4510 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a diamond like carbon (DLC) film, or the like can be formed.In addition, in a space which is formed with the first substrate 4001,the second substrate 4006, and the sealant 4005, a filler 4514 isprovided for sealing. It is preferable that a panel be packaged (sealed)with a protective film (such as a laminate film or an ultravioletcurable resin film) or a cover material with high air-tightness andlittle degasification so that the panel is not exposed to the outsideair, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon. Forexample, PVC (poly(vinyl chloride)), an acrylic resin, a polyimide, anepoxy resin, a silicone resin, PVB (poly(vinyl butyral)), or EVA(ethylene vinyl acetate) can be used. For example, nitrogen is used forthe filler.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate for a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also called anelectrophoretic display device (electrophoretic display) and hasadvantages in that it has the same level of readability as regularpaper, it has less power consumption than other display devices, and itcan be set to have a thin and light form.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent, each microcapsule containing first particleswhich are positively charged and second particles which are negativelycharged. By applying an electric field to the microcapsules, theparticles in the microcapsules move in opposite directions to each otherand only the color of the particles gathering on one side is displayed.Note that the first particles and the second particles each containpigment and do not move without an electric field. Moreover, the firstparticles and the second particles have different colors (which may becolorless).

Thus, an electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A mixture in which the above microcapsules are dispersed in a solvent isreferred to as electronic ink. This electronic ink can be printed on asurface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese.

As the electronic paper, a display device using a twisting ball displaysystem can be also used. The twisting ball display system refers to amethod in which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

FIG. 11 illustrates active matrix electronic paper as an embodiment of asemiconductor device. The electronic paper in FIG. 11 is an example of adisplay device using a twisting ball display system.

Between the first electrode layer 4030 connected to the transistor 4010and the second electrode layer 4031 provided for the second substrate4006, spherical particles 4613 each of which includes a black region4615 a, a white region 4615 b, and a cavity 4612 which is filled withliquid around the black region 4615 a and the white region 4615 b, areprovided. A space around the spherical particles 4613 is filled with afiller 4614 such as a resin. The second electrode layer 4031 correspondsto a common electrode (counter electrode). The second electrode layer4031 is electrically connected to a common potential line.

Note that in FIG. 9 to FIG. 11, a flexible substrate as well as a glasssubstrate can be used as the first substrate 4001 and the secondsubstrate 4006. For example, a plastic substrate havinglight-transmitting properties can be used. As plastic, afiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film can be used. Inaddition, a sheet with a structure in which an aluminum foil issandwiched between PVF films or polyester films can be used.

The insulating layer 4021 can be formed using an inorganic insulatingmaterial or an organic insulating material. Note that the insulatinglayer 4021 formed using a heat-resistant organic insulating materialsuch as an acrylic resin, a polyimide, a benzocyclobutene-based resin, apolyamide, or an epoxy resin is preferably used as a planarizinginsulating film In addition to such organic insulating materials, it ispossible to use a low-dielectric constant material (a low-k material), asiloxane based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like. The insulating layer may be formed bystacking a plurality of insulating films formed of these materials.

There is no particular limitation on the method for forming theinsulating layer 4021, and the insulating layer 4021 can be formed,depending on the material, by a sputtering method, a spin coatingmethod, a dipping method, spray coating, a droplet discharge method(e.g., an inkjet method), screen printing, offset printing, rollcoating, curtain coating, knife coating, or the like.

The display device displays an image by transmitting light from a lightsource or a display element. Therefore, the substrate and the thin filmssuch as the insulating film and the conductive film provided for thepixel portion where light is transmitted have light-transmittingproperties with respect to light in the visible-light wavelength range.

The first electrode layer and the second electrode layer (each of whichmay be called a pixel electrode layer, a common electrode layer, acounter electrode layer, or the like) for applying voltage to thedisplay element may have light-transmitting properties orlight-reflecting properties, which depends on the direction in whichlight is extracted, the position where the electrode layer is provided,the pattern structure of the electrode layer, and the like.

Any of the first electrode layer 4030 and the second electrode layer4031 can be formed using a light-transmitting conductive material suchas indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Any of the first electrode layer 4030 and the second electrode layer4031 can be formed using one or more kinds of materials selected frommetals such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium(Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt(Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper(Cu), and silver (Ag); alloys of these metals; and nitrides of thesemetals.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protective circuit is preferably formed using anon-linear element.

As described above, by using any of the transistors exemplified inEmbodiments 1 to 3, a highly-reliable semiconductor device can beprovided.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 7

A semiconductor device having an image sensor function for reading dataof an object can be formed with the use of the transistor whose exampleis described in any of Embodiments 1 to 3.

An example of a semiconductor device having an image sensor function isillustrated in FIG. 12A. FIG. 12A illustrates an equivalent circuit of aphoto sensor, and FIG. 12B is a cross-sectional view illustrating partof the photo sensor.

In a photodiode 602, one electrode is electrically connected to aphotodiode reset signal line 658, and the other electrode iselectrically connected to a gate of a transistor 640. One of a sourceand a drain of the transistor 640 is electrically connected to a photosensor reference signal line 672, and the other of the source and thedrain thereof is electrically connected to one of a source and a drainof a transistor 656. A gate of the transistor 656 is electricallyconnected to a gate signal line 659, and the other of the source and thedrain thereof is electrically connected to a photo sensor output signalline 671.

Note that in circuit diagrams in this specification, a transistorincluding an oxide semiconductor film is denoted by a symbol “OS” sothat it can be identified as a transistor including an oxidesemiconductor film The transistor 640 and the transistor 656 in FIG. 12Aare transistors each including an oxide semiconductor film.

FIG. 12B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photo sensor. The photodiode 602 functioning as asensor and the transistor 640 are provided over a substrate 601 (a TFTsubstrate) having an insulating surface. A substrate 613 is providedover the photodiode 602 and the transistor 640 with an adhesive layer608 provided therebetween. In addition, an insulating film 631, a firstinterlayer insulating layer 633, and a second interlayer insulatinglayer 634 are provided over the transistor 640.

Further, an electrode layer 645 b is provided in the same layer as thegate electrode 645 a of the transistor 640 so as to be electricallyconnected to the gate electrode 645 a of the transistor 640. Theelectrode layer 645 b is electrically connected to an electrode layer641 a through an opening provided in the insulating film 631 and thefirst interlayer insulating layer 633. The electrode layer 641 a iselectrically connected to an electrode layer 642 formed in the secondinterlayer insulating layer 634, and the electrode layer 642 iselectrically connected to the gate electrode 645 a through the electrodelayer 641 a; accordingly, the photodiode 602 is electrically connectedto the transistor 640.

The photodiode 602 is provided over the first interlayer insulatinglayer 633. In the photodiode 602, a first semiconductor layer 606 a, asecond semiconductor layer 606 b, and a third semiconductor layer 606 care sequentially stacked from the first interlayer insulating layer 633side, between an electrode layer 641 b formed over the first interlayerinsulating layer 633 and the electrode layer 642 formed over the secondinterlayer insulating layer 634.

In this embodiment, any of the transistors described in Embodiments 1 to3 can be applied to the transistor 640. Variation in electricalcharacteristics of the transistor 640 and the transistor 656 issuppressed and the transistor 640 and the transistor 656 areelectrically stable. Therefore, a highly-reliable semiconductor devicecan be provided as the semiconductor device of this embodiment describedin FIGS. 12A and 12B.

Here, a pin photodiode in which a semiconductor layer having a p-typeconductivity as the first semiconductor layer 606 a, a high-resistancesemiconductor layer (i-type semiconductor layer) as the secondsemiconductor layer 606 b, and a semiconductor layer having an n-typeconductivity as the third semiconductor layer 606 c are stacked isillustrated as an example.

The first semiconductor layer 606 a is a p-type semiconductor layer andcan be formed using an amorphous silicon film containing an impurityelement imparting a p-type conductivity. The first semiconductor layer606 a is formed by a plasma CVD method with use of a semiconductorsource gas containing an impurity element belonging to Group 13 (such asboron (B)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the be used.Further alternatively, an amorphous silicon film which does not containan impurity element may be formed, and then, an impurity element may beintroduced to the amorphous silicon film with use of a diffusion methodor an ion implantation method. Heating or the like may be conductedafter introducing the impurity element by an ion implantation method orthe like in order to diffuse the impurity element. In that case, as amethod of forming the amorphous silicon film, an LPCVD method, achemical vapor deposition method, a sputtering method, or the like maybe used. The first semiconductor layer 606 a is preferably formed tohave a thickness greater than or equal to 10 nm and less than or equalto 50 nm.

The second semiconductor layer 606 b is an i-type semiconductor layer(intrinsic semiconductor layer) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor layer 606 b, anamorphous silicon film is formed with use of a semiconductor source gasby a plasma CVD method. As the semiconductor source gas, silane (SiH₄)may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or thelike may be used. The second semiconductor layer 606 b may be formed byan LPCVD method, a vapor deposition method, a sputtering method, or thelike. The second semiconductor layer 606 b is preferably formed to havea thickness greater than or equal to 200 nm and less than or equal to1000 nm.

The third semiconductor layer 606 c is an n-type semiconductor layer andis formed using an amorphous silicon film containing an impurity elementimparting an n-type conductivity. The third semiconductor layer 606 c isformed by a plasma CVD method with use of a semiconductor source gascontaining an impurity element belonging to Group 15 (e.g., phosphorus(P)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like be used.Further alternatively, an amorphous silicon film which does not containan impurity element may be formed, and then, an impurity element may beintroduced to the amorphous silicon film with use of a diffusion methodor an ion implantation method. Heating or the like may be conductedafter introducing the impurity element by an ion injecting method or thelike in order to diffuse the impurity element. In that case, as a methodof forming the amorphous silicon film, an LPCVD method, a chemicalvapor, deposition method, a sputtering method, or the like may be used.The third semiconductor layer 606 c is preferably formed to have athickness greater than or equal to 20 nm and less than or equal to 200nm.

Any of the first semiconductor layer 606 a, the second semiconductorlayer 606 b, and the third semiconductor layer 606 c is not necessarilyformed using an amorphous semiconductor, and may be formed using apolycrystalline semiconductor, or a micro crystalline semiconductor (asemi-amorphous semiconductor: SAS).

The microcrystalline semiconductor belongs to a metastable state of anintermediate between amorphous and single crystalline when Gibbs freeenergy is considered. That is, the microcrystalline semiconductor is asemiconductor having a third state which is thermodynamically stable andhas a short range order and lattice distortion. Columnar-like orneedle-like crystals grow in a normal direction with respect to asubstrate surface. The Raman spectrum of microcrystalline silicon, whichis a typical example of a microcrystalline semiconductor, is located inlower wavenumber than 520 cm⁻¹, which represents a peak of the Ramanspectrum of single crystal silicon. That is, the peak of the Ramanspectrum of the microcrystalline silicon exists between 520 cm⁻¹ whichrepresents single crystal silicon and 480 cm⁻¹ which representsamorphous silicon. The semiconductor contains hydrogen or halogen of atleast 1 at. % to terminate a dangling bond. Moreover, microcrystallinesilicon is made to contain a rare gas element such as helium, neon,argon, or krypton to further enhance lattice distortion, wherebystability is increased and a favorable microcrystalline semiconductorfilm can be obtained.

The microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens ofmegahertz to several hundreds of megahertz or using a microwave plasmaCVD apparatus with a frequency of 1 GHz or more. Typically, themicrocrystalline semiconductor film can be formed by using a gasobtained by diluting SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄, withhydrogen Further, with a dilution of silicon hydride and hydrogen withone or plural kinds of rare gas elements selected from helium, neon,argon, and krypton, the microcrystalline semiconductor film can beformed. In that case, the flow ratio of hydrogen to silicon hydride is5:1 to 200:1, preferably 50:1 to 150:1, more preferably 100:1. Further,a hydrocarbon gas such as CH₄ or C₂H₆, a germanium gas such as GeH₄ orGeF₄, F₂, or the like may be mixed into the gas containing silicon.

In addition, since the mobility of holes generated by a photoelectriceffect is lower than that of electrons, a pin photodiode exhibits bettercharacteristics when a surface on the p-type semiconductor layer side isused as a light-receiving plane. Here, an example in which light 622received by the photodiode 602 from a surface of the substrate 601, overwhich the pin photodiode is formed, is converted into electric signalswill be described. Further, light from the semiconductor layer having aconductivity type opposite from that of the semiconductor layer on thelight-receiving plane is disturbance light; therefore, the electrodelayer 642 on the semiconductor layer having the opposite conductivitytype is preferably formed from a light-blocking conductive film. Notethat a surface of the n-type semiconductor layer side can alternativelybe used as the light-receiving plane.

For reduction of the surface roughness, an insulating layer functioningas a planarizing insulating film is preferably used for the firstinterlayer insulating layer 633 and the second interlayer insulatinglayer 634. Any of the first interlayer insulating layer 633 and thesecond interlayer insulating layer 634 can be formed using, for example,an organic insulating material such as a polyimide, an acrylic resin, abenzocyclobutene-based resin, a polyamide, or an epoxy resin. Inaddition to such organic insulating materials, it is possible to use asingle layer or stacked layers of a low-dielectric constant material (alow-k material), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like.

Any of the insulating film 631, the first interlayer insulating layer633, and the second interlayer insulating layer 634 can be formed usingan insulating material by a sputtering method, a spin coating method, adipping method, spray coating, a droplet discharge method (e.g., aninkjet method), screen printing, offset printing, roll coating, curtaincoating, knife coating, or the like depending on the material.

When the light 622 that enters the photodiode 602 is detected, data onan object to be detected can be read. Note that a light source such as abacklight can be used at the time of reading data on an object.

The transistor whose example is described in Embodiment 1, 2, or 3 canbe used as the transistor 640. The transistor including the oxidesemiconductor film which is highly purified by intentionally removingimpurities such as hydrogen, moisture, a hydroxyl group, or a hydride(also referred to as a hydrogen compound) and contains excessive oxygensupplied by oxygen doping treatment or the like has a suppressedvariation in the electrical characteristics and is electrically stable.Therefore, a highly-reliable semiconductor device can be provided.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

Embodiment 8

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof electronic appliances are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of electronic appliances each including the liquid crystaldisplay device described in the above embodiment will be described.

FIG. 13A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar cell 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader illustrated in FIG. 13Ahas a function of displaying various kinds of information (e.g., a stillimage, a moving image, and a text image) on the display portion, afunction of displaying a calendar, a date, the time, or the like on thedisplay portion, a function of operating or editing the informationdisplayed on the display portion, a function of controlling processingby various kinds of software (programs), and the like. Note that in FIG.13A, the charge and discharge control circuit 9634 has a battery 9635and a DCDC converter (hereinafter, abbreviated as a converter) 9636. Thesemiconductor device described in any of the above embodiments can beapplied to the display portion 9631, whereby a highly-reliableelectronic book reader can be provided.

In the case where a semi-transmissive liquid crystal display device or areflective liquid crystal display device is used as the display portion9631, use under a relatively bright condition is assumed; therefore, thestructure illustrated in FIG. 13A is preferable because power generationby the solar cell 9633 and charge for the battery 9635 are effectivelyperformed. Since the solar cell 9633 can be provided in a space (asurface or a rear surface) of the housing 9630 as appropriate, thebattery 9635 can be efficiently charged, which is preferable. When alithium ion battery is used as the battery 9635, there is an advantageof downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 13A will be described with reference toa block diagram in FIG. 13B. The solar cell 9633, the battery 9635, theconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are shown in FIG. 13B, and the battery 9635, the converter9636, the converter 9637, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the converter9636 so that the power has a voltage for charging the battery 9635.Then, when the power from the solar cell 9633 is used for the operationof the display portion 9631, the switch SW1 is turned on and the voltageof the power is raised or lowered by the converter 9637 to a voltageneeded for the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Next, operation in the case where power is not generated by the solarcell 9633 using external light is described. The voltage of poweraccumulated in the battery 9635 is raised or lowered by the converter9637 by turning on the switch SW3. Then, power from the battery 9635 isused for the operation of the display portion 9631.

Note that although the solar cell 9633 is described as an example of ameans for charge, the battery 9635 may be charged with another means. Inaddition, a combination of the solar cell 9633 and another means forcharge may be used.

FIG. 14A illustrates a laptop personal computer, which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. By applying the semiconductor device described in any of theabove embodiments to the display portion 3003, a highly-reliable laptoppersonal computer can be obtained.

FIG. 14B is a personal digital assistant (PDA), which includes a mainbody 3021 provided with a display portion 3023, an external interface3025, operation buttons 3024, and the like. A stylus 3022 is included asan accessory for operation. By applying the semiconductor devicedescribed in any of the above embodiments to the display portion 3023, ahighly-reliable personal digital assistant (PDA) can be obtained.

FIG. 14C illustrates an example of an electronic book reader. Forexample, an electronic book reader 2700 includes two housings, i.e., ahousing 2701 and a housing 2703. The housing 2701 and the housing 2703are combined with a hinge 2711 so that the electronic book reader 2700can be opened and closed with the hinge 2711 as an axis. With such astructure, the electronic book reader 2700 can operate like a paperbook.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the structure where different images are displayed ondifferent display portions, for example, the right display portion (thedisplay portion 2705 in FIG. 14C) displays text and the left displayportion (the display portion 2707 in FIG. 14C) displays images. Byapplying the semiconductor device described in any of the aboveembodiments to the display portions 2705 and 2707, the electronic bookreader 2700 can have high reliability.

FIG. 14C illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. With the operation keys 2723, pages can be turned. Notethat a keyboard, a pointing device, or the like may also be provided onthe surface of the housing, on which the display portion is provided.Furthermore, an external connection terminal (an earphone terminal, aUSB terminal, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the electronic book reader 2700 may have a functionof an electronic dictionary.

The electronic book reader 2700 may have a structure capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired book data or the like can be purchased anddownloaded from an electronic book server.

FIG. 14D illustrates a mobile phone, which includes two housings, i.e.,a housing 2800 and a housing 2801. The housing 2801 includes a displaypanel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, acamera lens 2807, an external connection terminal 2808, and the like. Inaddition, the housing 2800 includes a solar cell 2810 having a functionof charge of the mobile phone, an external memory slot 2811, and thelike. An antenna is incorporated in the housing 2801. By applying thesemiconductor device described in any of the above embodiments to thedisplay panel 2802, a highly-reliable mobile phone can be obtained.

Further, the display panel 2802 is provided with a touch panel. Aplurality of operation keys 2805 which is displayed as images isillustrated by dashed lines in FIG. 14D. Note that a boosting circuit bywhich a voltage output from the solar cell 2810 is increased to besufficiently high for each circuit is also included.

In the display panel 2802, the display direction can be appropriatelychanged depending on a usage pattern. Further, the mobile phone isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus it can be used as a video phone. The speaker 2803and the microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Furthermore, thehousings 2800 and 2801 which are developed as illustrated in FIG. 14Dcan overlap with each other by sliding; thus, the size of the mobilephone can be decreased, which makes the mobile phone suitable for beingcarried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a largeamount of data can be stored by inserting a storage medium into theexternal memory slot 2811 and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 14E illustrates a digital video camera which includes a main body3051, a display portion A 3057, an eyepiece portion 3053, an operationswitch 3054, a display portion B 3055, a battery 3056, and the like. Byapplying the semiconductor device described in any of the aboveembodiments to the display portion A 3057 and the display portion B3055, a highly-reliable digital video camera can be obtained.

FIG. 14F illustrates an example of a television device. In a televisionset 9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605. By applying the semiconductor devicedescribed in any of the above embodiments to the display portion 9603,the television set 9600 with high reliability can be obtained.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

This application is based on Japanese Patent Application serial no.2010-100237 filed with Japan Patent Office on Apr. 23, 2010, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

10: plasma apparatus; 11: substrate supply chamber; 12: load lockchamber; 13: transfer chamber; 14: cassette port; 15: vacuum chamber;16: ICP coil; 17: gas flow path; 18: first high-frequency power source;19: substrate stage; 20: process substrate; 21: second high-frequencypower source; 22: automatic pressure control valve; 23: turbo molecularpump; 24: dry pump; 100: substrate; 102: insulating film; 102 a:insulating film; 102 b: insulating film; 103 a: mask; 103 b: mask; 104a: source electrode; 104 b: drain electrode; 106: oxide semiconductorfilm; 108: oxide semiconductor film; 110: gate insulating film; 110 a:gate insulating film; 110 b: gate insulating film; 112; gate electrode;112 a: gate electrode; 112 b: electrode; 114: insulating film; 120:transistor; 130: transistor; 140: transistor; 150: transistor; 151:insulating film; 152: insulating film; 154: electrode; 156: wiring; 160:transistor; 164: capacitor; 180: oxygen; 180 a: oxygen; 180 b: oxygen;180 c: oxygen; 200: substrate; 206: element isolation insulating film;208: gate insulating film; 210: gate electrode; 216: channel formationregion; 220: impurity region; 224: metal compound region; 228:insulating film; 230: insulating film; 240: transistor; 601: substrate;602: photodiode; 606 a: semiconductor layer, 606 b: semiconductor layer;606 c: semiconductor layer; 608: adhesive layer; 613: substrate; 622:light; 631: insulating film; 633: interlayer insulating layer; 634:interlayer insulating layer, 640: transistor, 641: electrode layer; 642:electrode layer; 643: conductive layer; 645: gate electrode; 656:transistor; 658: photodiode reset signal line; 659: gate signal line;671: photo sensor output signal line; 672: photo sensor reference signalline; 2700: electronic book reader; 2701: housing; 2703: housing; 2705:display portion; 2707: display portion; 2711: hinge; 2721: power switch;2723: operation key; 2725: speaker, 2800: housing, 2801: housing; 2802:display panel; 2803: speaker, 2804: microphone; 2805: operation key;2806: pointing device; 2807: camera lens; 2808: external connectionterminal; 2810: solar cell; 2811: external memory slot; 3001: main body;3002: housing; 3003: display portion; 3004: keyboard; 3021: main body;3022: stylus; 3023: display portion; 3024: operation button; 3025:external interface; 3051: main body; 3053: eyepiece portion; 3054:operation switch; 3055: display portion B; 3056: battery; 3057: displayportion A; 4001: substrate; 4002: pixel portion; 4003: signal linedriver circuit; 4004: scan line driver circuit; 4005: sealant; 4006:substrate; 4008: liquid crystal layer; 4010: transistor; 4011:transistor; 4013: liquid crystal element; 4015: connection terminalelectrode; 4016: terminal electrode; 4018: FPC; 4019: anisotropicconductive film; 4021: insulating layer; 4030: electrode layer, 4031:electrode layer; 4032: insulating film; 4033: insulating film; 4035:spacer; 4510: partition wall; 4511: electroluminescent layer; 4513:light-emitting element; 4514: filler; 4612: cavity; 4613: sphericalparticle; 4614: filler, 4615 a: black region; 4615 b: white region;9600: television set; 9601: housing; 9603: display portion; 9605: stand;9630: housing; 9631: display portion; 9632: operation key; 9633: solarcell; 9634: charge and discharge control circuit; 9635: battery; 9636:converter; 9637: converter.

The invention claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming a first insulating film;forming a source electrode, a drain electrode, and an oxidesemiconductor film over the first insulating film, wherein the oxidesemiconductor film is electrically connected to the source electrode andthe drain electrode; performing first oxygen doping treatment on theoxide semiconductor film; forming a second insulating film over theoxide semiconductor film; performing second oxygen doping treatment onthe second insulating film; and forming a gate electrode over the oxidesemiconductor film with the second insulating film interposedtherebetween.
 2. The method according to claim 1, wherein the oxidesemiconductor film is formed over the source electrode and the drainelectrode.
 3. The method according to claim 1, wherein the sourceelectrode and the drain electrode are formed over the oxidesemiconductor film.
 4. The method according to claim 1, furthercomprising a step of forming a third insulating film over the gateelectrode.
 5. The method according to claim 1, further comprising thestep of forming a fourth insulating film, wherein the first insulatingfilm is formed over the fourth insulating film, and wherein the firstinsulating film comprises a component which is the same as that of theoxide semiconductor film.
 6. The method according to claim 1, furthercomprising the step of forming a fourth insulating film, wherein thefirst insulating film is formed over the fourth insulating film, whereinthe first insulating film comprises a component which is the same asthat of the oxide semiconductor film, and wherein the fourth insulatingfilm comprises a component which is different from that of the oxidesemiconductor film.
 7. The method according to claim 1, furthercomprising the step of forming a fourth insulating film over the secondinsulating film, wherein the second insulating film comprises acomponent which is the same as that of the oxide semiconductor film. 8.The method according to claim 1, further comprising the step of forminga fourth insulating film over the second insulating film, wherein thesecond insulating film comprises a component which is the same as thatof the oxide semiconductor film, and wherein the fourth insulating filmcomprises a component which is different from that of the oxidesemiconductor film.
 9. The method according to claim 1, wherein theoxide semiconductor film comprises gallium as a component.
 10. Themethod according to claim 1, wherein the oxide semiconductor filmcomprises indium, gallium, and zinc.
 11. A method for manufacturing asemiconductor device, comprising the steps of: forming a first galliumoxide film; forming a source electrode, a drain electrode, and an oxidesemiconductor film over the first gallium oxide film, wherein the oxidesemiconductor film is electrically connected to the source electrode andthe drain electrode; performing first oxygen doping treatment on theoxide semiconductor film; forming a second gallium oxide film over theoxide semiconductor film; performing second oxygen doping treatment onthe second gallium oxide film; and forming a gate electrode over theoxide semiconductor film with the second gallium oxide film interposedtherebetween.
 12. The method according to claim 11, wherein the oxidesemiconductor film is formed over the source electrode and the drainelectrode.
 13. The method according to claim 11, wherein the sourceelectrode and the drain electrode are formed over the oxidesemiconductor film.
 14. The method according to claim 11, furthercomprising a step of forming a third insulating film over the gateelectrode.
 15. The method according to claim 11, further comprising thestep of forming a first insulating film, wherein the first gallium oxidefilm is formed over the first insulating film.
 16. The method accordingto claim 11, further comprising the step of forming a first insulatingfilm, wherein the first gallium oxide film is formed over the firstinsulating film, and wherein the first insulating film comprises acomponent which is different from that of the oxide semiconductor film.17. The method according to claim 11, further comprising the step offorming a second insulating film over the second gallium oxide film. 18.The method according to claim 11, further comprising the step of forminga second insulating film over the second gallium oxide film, wherein thesecond insulating film comprises a component which is different fromthat of the oxide semiconductor film.
 19. The method according to claim11, wherein the oxide semiconductor film comprises gallium as acomponent.
 20. The method according to claim 11, wherein the oxidesemiconductor film comprises indium, gallium, and zinc.
 21. The methodaccording to claim 11, further comprising the step of performing heattreatment on the oxide semiconductor film.
 22. A method formanufacturing a semiconductor device, comprising the steps of: forming afirst insulating film; forming a source electrode, a drain electrode,and an oxide semiconductor film over the first insulating film, whereinthe oxide semiconductor film is electrically connected to the sourceelectrode and the drain electrode; performing heat treatment on theoxide semiconductor film; performing first oxygen doping treatment onthe oxide semiconductor film; forming a second insulating film over theoxide semiconductor film; performing second oxygen doping treatment onthe second insulating film; and forming a gate electrode over the oxidesemiconductor film with the second insulating film interposedtherebetween.
 23. The method according to claim 22, wherein the oxidesemiconductor film is formed over the source electrode and the drainelectrode.
 24. The method according to claim 22, wherein the sourceelectrode and the drain electrode are formed over the oxidesemiconductor film.
 25. The method according to claim 22, furthercomprising a step of forming a third insulating film over the gateelectrode.
 26. The method according to claim 22, wherein the firstinsulating film comprises a fourth insulating film and a fifthinsulating film over the fourth insulating film, and wherein the fifthinsulating film comprises a component which is the same as that of theoxide semiconductor film.
 27. The method according to claim 22, whereinthe first insulating film comprises a fourth insulating film and a fifthinsulating film over the fourth insulating film, wherein the fifthinsulating film comprises a component which is the same as that of theoxide semiconductor film, and wherein the fourth insulating filmcomprises a component which is different from that of the oxidesemiconductor film.
 28. The method according to claim 22, wherein thesecond insulating film comprises a sixth insulating film and a seventhinsulating film over the sixth insulating film, and wherein the sixthinsulating film comprises a component which is the same as that of theoxide semiconductor film.
 29. The method according to claim 22, whereinthe second insulating film comprises a sixth insulating film and aseventh insulating film over the sixth insulating film, wherein thesixth insulating film comprises a component which is the same as that ofthe oxide semiconductor film, and wherein the seventh insulating filmcomprises a component which is different from that of the oxidesemiconductor film.
 30. The method according to claim 22, wherein theoxide semiconductor film comprises gallium as a component.
 31. Themethod according to claim 22, wherein the oxide semiconductor filmcomprises indium, gallium, and zinc.